Use issue templates and add diagram, theme and syntax proposal issue forms

.github/ISSUE_TEMPLATE/bug_report.yml

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+name: Bug Report
+description: Create a report to help us improve
+labels:
+  - 'Status: Triage'
+  - 'Type: Bug / Error'
+
+body:
+  - type: markdown
+    attributes:
+      value: |-
+        ## Security vulnerabilities
+        Please refer our [Security Policy](https://github.com/mermaid-js/.github/blob/main/SECURITY.md) and report to keep vulnerabilities confidential so we can release fixes first.
+
+        ## Before you submit...
+        We like to help you, but in order to do that should you make a few things first:
+
+        - Use a clear and concise title
+        - Fill out the text fields with as much detail as possible.
+        - Never be shy to give us screenshots and/or code samples. It will help!
+  - type: textarea
+    attributes:
+      label: Description
+      description: Give a clear and concise description of what the bug is.
+      placeholder: When I do ... does ... happen.
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Steps to reproduce
+      description: Give a step-by-step example on how to reproduce the bug.
+      placeholder: |-
+        1. Do this
+        2. Do that
+        3. ...
+        4. Bug!
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Screenshots
+      description: If applicable, add screenshots to help explain your issue.
+  - type: textarea
+    attributes:
+      label: Code Sample
+      description: |-
+        If applicable, add the code sample or a link to the [Live Editor](https://mermaid.live).
+        Any text pasted here will be rendered as a Code block.
+      render: text
+  - type: textarea
+    attributes:
+      label: Setup
+      description: |-
+        Please fill out the below info.
+        Note that you only need to fill out one and not both sections.
+      value: |-
+        **Desktop**
+
+        - OS and Version: [Windows, Linux, Mac, ...]
+        - Browser and Version: [Chrome, Edge, Firefox]
+
+        **Smartphone**
+
+        - Device: [Samsung, iPhone, ...]
+        - OS and Version: [Android, iOS, ...]
+        - Browser and Version: [Chrome, Safari, ...]
+  - type: textarea
+    attributes:
+      label: Additional Context
+      description: Anything else to add?

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: GitHub Discussions
+    url: https://github.com/mermaid-js/mermaid/discussions
+    about: Ask the Community questions or share your own graphs in our discussions.
+  - name: Security Vulnerability
+    url: https://github.com/mermaid-js/.github/blob/main/SECURITY.md
+    about: Report security issues confidentially.
+  - name: Slack
+    url: https://join.slack.com/t/mermaid-talk/shared_invite/enQtNzc4NDIyNzk4OTAyLWVhYjQxOTI2OTg4YmE1ZmJkY2Y4MTU3ODliYmIwOTY3NDJlYjA0YjIyZTdkMDMyZTUwOGI0NjEzYmEwODcwOTE
+    about: Join our Community on Slack for Help and a casual chat.
+  - name: Documentation
+    url: https://mermaid-js.github.io
+    about: Read our documentation for all that Mermaid.js can offer.
+  - name: Live Editor
+    url: https://mermaid.live
+    about: Try the live editor to preview graphs in no time.

.github/ISSUE_TEMPLATE/diagram_proposal.yml

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+name: Diagram Proposal
+description: Suggest a new Diagram Type to add to Mermaid.
+labels:
+  - 'Status: Triage'
+  - 'Type: Enhancement'
+
+body:
+  - type: markdown
+    attributes:
+      value: |-
+        ## Before you submit...
+        First of all, thank you for proposing a new Diagram to us.
+        We are always happy about new ideas to improve Mermaid.js wherever possible.
+
+        To get the fastest and best response possible, make sure you do the following:
+
+        - Use a clear and concise title
+        - Fill out the text fields with as much detail as possible.
+        - Never be shy to give us screenshots and/or code samples. It will help!
+  - type: textarea
+    attributes:
+      label: Proposal
+      description: A clear and concise description of what should be added to Mermaid.js.
+      placeholder: Mermaid.js should add ... because ...
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Use Cases
+      description: If applicable, give some use cases for where this diagram would be useful.
+      placeholder: The Diagram could be used for ...
+  - type: textarea
+    attributes:
+      label: Screenshots
+      description: If applicable, add screenshots to show possible examples of how the diagram may look like.
+  - type: textarea
+    attributes:
+      label: Code Sample
+      description: |-
+        If applicable, add a code sample for how to implement this new diagram.
+        The text will automatically be rendered as JavaScript code.
+      render: javascript

.github/ISSUE_TEMPLATE/syntax_proposal.yml

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+name: Syntax Proposal
+description: Suggest a new Syntax to add to Mermaid.js.
+labels:
+  - 'Status: Triage'
+  - 'Type: Enhancement'
+
+body:
+  - type: markdown
+    attributes:
+      value: |-
+        ## Before you submit...
+        First of all, thank you for proposing a new Syntax to us.
+        We are always happy about new ideas to improve Mermaid.js wherever possible.
+
+        To get the fastest and best response possible, make sure you do the following:
+
+        - Use a clear and concise title
+        - Fill out the text fields with as much detail as possible. Examples are always welcome.
+        - Never be shy to give us screenshots and/or code samples. It will help!
+  - type: textarea
+    attributes:
+      label: Proposal
+      description: A clear and concise description of what Syntax should be added to Mermaid.js.
+      placeholder: Mermaid.js should add ... because ...
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Example
+      description: If applicable, provide an example of the new Syntax.
+  - type: textarea
+    attributes:
+      label: Screenshots
+      description: If applicable, add screenshots to show possible examples of how the theme may look like.

.github/ISSUE_TEMPLATE/theme_proposal.yml

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+name: Theme Proposal
+description: Suggest a new theme to add to Mermaid.js.
+labels:
+  - 'Status: Triage'
+  - 'Type: Enhancement'
+
+body:
+  - type: markdown
+    attributes:
+      value: |-
+        ## Before you submit...
+        First of all, thank you for proposing a new Theme to us.
+        We are always happy about new ideas to improve Mermaid.js wherever possible.
+
+        To get the fastest and best response possible, make sure you do the following:
+
+        - Use a clear and concise title
+        - Fill out the text fields with as much detail as possible. Examples are always welcome!
+        - Never be shy to give us screenshots and/or code samples. It will help!
+  - type: textarea
+    attributes:
+      label: Proposal
+      description: A clear and concise description of what theme should be added to Mermaid.js.
+      placeholder: Mermaid.js should add ... because ...
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Colors
+      description: |-
+        A detailed list of the different colour values to use.
+        A list of currently used variable names can be found [here](https://mermaid-js.github.io/mermaid/#/theming?id=theme-variables-reference-table)
+      placeholder: |-
+        - background: #f4f4f4
+        - primaryColor: #fff4dd
+        - ...
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Screenshots
+      description: If applicable, add screenshots to show possible examples of how the theme may look like.