First pass at autodoc support

[tomchristie]

I've started working on an autodoc extension for use with mkdocs.

This pull request demonstrates using it with the httpx.request function, just for visibility of where I think we need to go here.

Bits to do:

• Nested code blocks in the docstring are not rendering. (I tried installing the pymdownx.superfences extension, which didn't appear to resolve the issue, either. *Any help with resolving this would be much appreciated.**) - Resolved
• Need to look at rendering classes, not just functions. More complicated. There's some limited support for this, but will need work. - Resolved
• Rendering docs for instance attributes on classes. This is more awkward to do than methods & properties, since we can only figure out instance attributes by grepping the code in the __init__.
• Ensure async methods get an "async" marker. We're not rendering any of these yet in this PR, so not currently an issue.
• I'm using a dd element as a bit of a styling hack, as it gives me a nicely indented block for the docstring. Probably ought to be a div with an appropriate class, or some other appropriate block element, with styling applied ideally by mkdocs-material, rather than having to add custom styling for it. - Resolved
• It'd be really nice to keep the typing information out of the main display, but have contextual information pop up against parameters. We can treat this as a nice-to-have-later.

[tomchristie]

Here's how we're looking now...

I think we're possibly(?) far along with this now that we could consider pulling it in, and then making incremental improvements as we go. It's a bit tricky trying to judge how much typing information to include in docstring descriptions when a parameter can accept a number of different types - I've not been terribly precise about that yet - would be happy to either try to clean anything up comprehensively in this PR, or take this as a starting point.

Things that would be nice in the future:

• Ensure that mkautodocs adds an <em>async</em> in front of any async methods/functions.
• Add contextual typing information to parameters. (Keep it hidden by default, but all hover or click popovers)
• Support documenting instance attributes. (Infer self.<some_valid_variable_name_pattern = from docstrings, and use any leading comment blocks prior to that as the docs.)

[florimondmanca]

@tomchristie Do you think this is ready for review, and potentially merging in?

[tomchristie]

@florimondmanca Yes I think so. Obvs we'll have some work to do on:

• Fleshing things out more.
• Figuring out a good approach to how much type information to talk about in the docstrings
• Figuring out a good approach to doc string styling.
• Improving functionality, eg. include contextual type information on hover/click or similar.

But from my POV I think it's probably worth us getting in, and then iterating on.

[florimondmanca]

Yup, looks good to me! Just a few possible typos.

[florimondmanca]

I fixed the few typos from my last review, merging in now! 🎉

[tomchristie]

Thanks! 😃