docs: generate Starlark domain markup instead of regular markdown

[rickeylev]

This switches our doc generation over to using the Starlark domain markup from the sphinx_stardoc plugin instead of using regular markdown. This allows the docs generated from code to better integrate with each other and other parts of the doc site.

Overview of changes:

• Makes the doc paths under the API directory more directly mirror their actual location. e.g. moves "defs.md" -> "python/defs.md". This is so the //tools doc entries have a more natural location, but can also be used for our other top-level directories.
• Adds API docs for some of the well known targets we have. These aren't automatically generated, but use the Starlark domain markup, so integrate nicely with everything.
• Ensures default values are parsable as Python expressions. Stardoc returns values like "" or 'Label(*, "//bar")' in some cases for the default value of args/attrs.
• Ensures function signatures don't crash doc rendering. Stardoc gives bad/incomplete information, so reconstructing the original signature of a function is tricky.
• Allows references flags using leading slashes and a value, e.g. --foo=bar. This makes it more natural to write while cross referencing to the flag.
• Implements {any} xref resolution. It was just totally broken before.
• Adds some additional bzl files that get documented.
• Adds some more Bazel external references.
• Fixes some missing bzl_library dependencies.
• A few minor QoL improvements to the docs dev server:
  • Print the serving directory when CTRL+C is received. This makes it easier to find the raw files that are being generated.
  • Fix an error during shutdown about an unterminated generator.
• The sphinx_stardocs.footer arg is removed. This was always just a hack to get extra link targets into the generated bzl docs. It's no longer needed when the bzl domain is used.
• Using @repo//pkg:file.bzl%Name syntax is supported in type expressions (e.g. :type: option or {type} role) by quoting the label. The quoting is necessary because, under the hood, the expressions are parsed as Python.
• Objects directives support an :origin-key directive. This records the label identity that Bazel sees for an object (as from the Stardoc origin_key field). The markdown generate doesn't generate this for everything yet because some things are documented twice (e.g. py_binary in defs.bzl and py_binary.bzl), which would cause a crash (type things trying to define the same id).
• Add * and ** to var-args and var-kwargs in signatures.
• Allow providers to be refered to using the type role. This allows providers to be referenced in :type: directives (e.g. in a provider field).

[aignas]

The docs look much better, thanks!