[docs] Generate linting docs from source

[jugglinmike]

When new rules have been added to WPT's "lint" tool, the corresponding
documentation has not always been updated [1] [2] [3]. Automatically
generating documentation from source code helps avoid this state and the
confusion it can cause contributors.

Introduce a pattern for defining linting rules. Rely on the new
structure during documentation generation to automatically create a
listing of all available linting rules.

The new structure centralizes information about the rules (name,
description, and instructions for correcting infractions). The process
honors the existing convention of Markdown within descriptions.

Although the Sphinx documentation generator includes a built-in
extension for generating documentation from Python source code, the
output of that extension is designed to document Python primitives such
as functions and classes. Such a format is inappropriate for this case
because the users of the linting tool do not interact with the internals
in this way. Define a custom docutils directive to tailor the
documentation to the needs of its audience.

[1] web-platform-tests/wpt#5299
[2] web-platform-tests/wpt#10501
[3] web-platform-tests/wpt#11479

---

This is intended to resolve gh-4.

@zcorpan @jgraham @gsnedders @marcoscaceres I'm demoing this with two rules,
but I'll be happy to migrate the rest of the rules to this format if the
general solution looks good to you. Here's a screen shot of the rendered
output:

What do you think?

[zcorpan]

I think this is a good approach 👍

It looks like lint/tests/test_file_lints.py may need to be changed as part of this.

[marcoscaceres]

I also really like this approach. Having the clear separation between the explanation and how to fix is really nice.

[jgraham]

This approach looks good. Please endeavour to land the lint changes upstream rather than in a docs branch, even if the motivating documentation isn't ready yet, otherwise there's a severe chance of conflicts which in this case would likely lead to regressing whole lint rules.

[jugglinmike]

Sure thing, @jgraham. Here's a patch that's scoped to the linter's source code:

web-platform-tests/wpt#16268