Migrate API docs to mkdocstrings

[anuraaga]

Summary

Migrates API docs from autodoc to mkdocstrings. I was hoping to be able to use an httpx inventory file in some documentation and noticed there seems to be interest in it from the attached issue so tried sending a PR.

I have made tweaks such as to the CSS to make it as close to the current site as possible. Though it could also make sense to have the full TOC.

The biggest difference in rendering is the docs don't have a inlet and are rendered directly. There are some other differences like font size and not showing the word class (mkdocstrings may allow it in the future as they have an insiders-only feature for it right now).

I did leave the source code previews since while very different from current, being collapsed they seemed safe to keep.

To really render using mkdocstrings, i.e. rendering out parameters in a structured way, the docstrings themselves would need to be updated to one of the supported styles like Google, numpy, or sphinx. This PR doesn't change that though to be smaller and still be quite close to what's already there, while also providing the inventory file.

To test, I run

uv run --with mkdocs --with mkdocs-material --with mkautodoc --with 'mkdocstrings[python]' mkdocs build
open site/index.html

Fixes #3145

 uvx sphobjinv suggest site/objects.inv as_rst

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Cannot infer intersphinx_mapping from a local objects.inv.

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Project: HTTPX
Version: 0.0.0

164 objects in inventory.

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No results found with score at/above current threshold of 75.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[anuraaga]

@lovelydinosaur Any interest in this? If not will go ahead and close this. Thanks.

[pollier]

Hello, it would be great to be able to use objects.inv, generated by mkdocstring to have auto cross reference in project