Use MkDocs autodoc to generate documentation

[yifanmai]

In particular, we want to generate documentation for built-in scenarios, models, adapters, etc. Auto-generating the documentation will make it easier to keep things in sync.

[yifanmai]

Ended up using MkDocs instead of Sphinx because:

• Native support for Markdown, which we already use (Sphinx prefers ReStructuredText and needs an extension for Markdown)
• Better developer ergonomics (e.g. hot reloading server)

[yifanmai]

Running into a weird issue where using Google style docstrings with mkdocstrings causes it to spuriously create sections where it shouldn't e.g.

Example:

    This should be code, not a section

The workaround is to delete the trailing colon.

[yifanmai]

Here's what the undesired section rendering looks like:

I deleted the colons for now. I'll add them back after figuring out how to disable the section rendering (maybe there's a config flag somewhere).

[yifanmai]

The regular expression that creates the undesired sections / admonitions is here. As far as I can tell, there is no way to disable it. The workarounds are to avoid the regex e.g. by adding **bold** markdown or other punctuation to the header.

[yifanmai]

We want dataclass documentation as well, as discussed in #1117

[yifanmai]

As noted in #1209, we need to add more schemas for dataclasses e.g. MetricName, Run, AdapterSpec, ScenarioState, etc.

[yifanmai]

MkDocs is used now so I will close this issue. There's more documentation to write, but that should be in different issues.