Add full API docs and improve the UX of navigating them

[hmellor]

Changes:

• Ported from sphinx.ext.autodoc and sphinx.ext.autosummary to autodoc2:
  • autodoc2 supports myst (markdown) so we can stop writing rst formatted docstrings
  • The entire codebase is documented in API Reference
  • All of these docs can be cross-referenced throughout the docs
• Updated the API Reference index to include all the important classes/functions we already singled out on the individual pages we had before (reduces users having to click around and shows a more global picture of the APIs we expect them to be using)
• autodoc2 statically analyses the code so:
  • No more mocked import issues outside of docs/
  • Greatly reduced requirements/docs.txt
  • RTD builds are slower because this analysis takes time
• sphinx.ext.linkcode no longer requires vllm to be importable

This should help JSON CLI args document themselves (such as in #17420)

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[hmellor]

I almost have a patch that replaces autodoc and handles API documentation much better in general.

Let's hold off on merging this until that is ready

[DarkLight1337]

Hmm it's still a bit difficult to navigate to the API docs of specific modules. Can we make Offline Inference, vLLM Engines etc. sub-pages under Summary, and similar for the "vllm" auto-generated pages?

[hmellor]

Ok, currently working on improving the left sidebar so that the entire API can be navigated properly

[DarkLight1337]

This looks good now. Thanks for improving the dev experience!

[DarkLight1337]

To avoid breaking stuff, let's wait for #17607 to unblock CI