[docs] Add the new docs CI checks

[colleenmcginnis]

Adds the new docs CI checks. This will enable a preview to build our new Markdown-based docs.

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

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• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[marciw]

approving despite the buildkite failure -- see this comment

[marciw]

@pquentin you mentioned here that the Buildkite CI is currently broken. Any way we can get around that for now? (this PR needs to be merged before we tackle #2806)

[miguelgrinberg]

@marciw The problem with our build is unrelated to your changes, as far as I can see. If this is blocking you from completing your migration let me know, as I can force-merge this for you.

[marciw]

@marciw The problem with our build is unrelated to your changes, as far as I can see. If this is blocking you from completing your migration let me know, as I can force-merge this for you.

@miguelgrinberg thank you! Yes, that would be great so we can get everything in place for 9.x. Can you force-merge this one? Then I can generate a preview in #2806 (which I'll need to check before we force-merge that one). 🙏

[miguelgrinberg]

@marciw Merged! I think by the time you are ready to merge the other one the build will be again passing, but feel free to ping me if not.