Add doc rendering via starlight

[steveklabnik]

This change adds a new docs deployment to /starlight/, and that's rendering the documentation via Starlight: https://starlight.astro.build/

mkdocs is no longer being maintained, and we'd like to move away from it for that reason. We've chosen starlight for two main reasons: first, it has a goal of minimizing JavaScript on the front end, and that aligns with our priorities. Second, it is based on Astro.js, which would also be a good tool for writing a basic marketing site. The overall sentiment from community in the Discord was leaning towards Astro when we were talking about this.

After a few days of checking out the new docs and making sure that things are good, we can move to making this the default.

Part of #7959.

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This PR is not actually ready to merge as-is. The reason is this: I wanted to demonstrate the actual diff of this PR, and so I modified the files in docs. However, that means they aren't going to properly render with mkdocs. However, this is actually easy to fix: there's this line in the config:

const docsDir = fileURLToPath(new URL('../../docs', import.meta.url));

This let us use the existing docs dir. I can easily instead copy the files to web/docs/src/docs, and modify them there, and then we'll have both copies while we try it out. I'm not sold on if we should keep the configuration to use docs at the top level permanently or move the files into the actual site in the long run, but before this gets merged, we should do the duplication just to try it out. If anything modifies those files before we make this permanent, I'll make sure to reconcile things.

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[steveklabnik]

fun story: this line cost me ~2 hours of debugging: the markdown implementations differ ever so slightly, and without this blank line, it doesn't recognize git-autocrlf as a link target.

[steveklabnik]

I probably want to make this more real before merge.

All commits are now correctly formatted. Thank you for your contribution!

[steveklabnik]

this should be switched to the bird before merge

[steveklabnik]

Oh also, I based this off of @senekor 's work over at main...senekor/starlight-docs, so would love a review from him especially.

[martinvonz]

Is this a markdown table? It looks like we're sacrificing readability in the GitHub-rendered form for better rendering with starlight. I suppose there should be little reason to look at GitHub's rendering, especially if we can get renderings from PRs with starlight, so we can review the rendered output. Oh, perhaps these new titles will not be rendered as a heading at the top of the page so this is more like a directive to starlight?

[PhilipMetzger]

I think preserving the title in GH's rendering is worth it, since it simplifies contributions.

Oh, perhaps these new titles will not be rendered as a heading at the top of the page so this is more like a directive to starlight?

Yes, that's the main reason.