docs: add new website using Astro Starlight

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 📝 Docs: Standardize slogan as something without "simple" #5065; fixes 📝 Docs: Overhaul website generation #5207
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Creates a new website under docs-next/ using Astro Starlight. It contains roughly the same documentation as before, just split up into a more modern website with a navigation sidebar and a table-of-contents on each page. Its base route is next/, like https://mochajs.org/next.

🌟 This is deployed to Netlify on: https://mocha-docs-next.netlify.app.

"Feature parity" tasks that I took for this PR:

• Set up a Starlight site on a /next base URI
• Create visual theme based on the existing brand colors
  • Port over the top-of-page animated logo
  • Port over the footer
  • Polish dark mode
  • Fix all auto-detectable accessibility issues
• Separate existing headings and content into structured content
  • Reorder cli.mdx to match actual CLI output and sections
  • Add new interfaces/third-party page to match reporters/third-party
• Update inline links to use the new structure
• Update block quotes and other emphasized lines to admonitions
• Add build script to generate sponsors data

After this is merged, we'll want to:

1. Share it out for early interest & feedback
2. Wait a while
3. Deploy this on mochajs.org/next, with existing routes such as /api not being impacted
4. Eventually switch this site to be the root page, with the old one under something like /old
   • Needed: JavaScript-based redirects for old # headings
   • Needed: Migrate the existing /api old JSDoc & docdash generation (or an equivalent)
   • Needed: Migrate the existing /examples/test page from the old docs

Later-on out-of-scope items that I think would be good:

• Add a nicer preview / thumbnail
• Fill in missing content
  • Move content from the wiki that should be here, such as third-party interfaces and reporters
  • Move mochajs/mocha/tree/main/example/config to the examples repo
• Refresh Editor Plugins
• Revamp the sponsors & backers lists to be more based on total donations
• Revamp the content to be more clear, organized, and up-to-date with modern practices
  • Stop discouraging arrow functions in features/arrow-functions
• Run an accessibility audit on link titles and image descriptions
• Update screenshots
• Update out-of-date information
  • Editor plugins, reporters
  • Screenshots of browsers
  • Third-party examples
• Update version notices older than 8.0.0 (see existing issue)
• Onboard /api to something new like TypeDoc
• 404 page

Astro/Starlight issues (pending https://discord.com/channels/830184174198718474/1070481941863878697/1303801639785726045):

• To be filed: Suggest ComponentProps<typeof Default> in overriding components
• To be filed: Asides that are empty: don't add extra vertical interior margin

💖

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[mark-wiemer]

Looks pretty darn good!

[JoshuaKGoldberg]

Thanks so much for the thorough review @mark-wiemer!

For all the suggestions to Astro/infra areas and introduced issues: 👍 agreed, applied.

For the clarity, proofreading, and other existing content changes suggestions: I propose we tackle those in >1 followup PR. Some of them are more subjective and might take some time for discussion. I think it'd be valuable to get this PR merged in the interim. WDYT?