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 "<function foo>" 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).