sphinxdocs: add docs; support sources from other directories

[rickeylev]

Documents how to use Sphinx syntax when writing docs. There's a variety of features the sphinx_bzl plugin enables, but without docs, they're somewhat hard to discover and figure out how to use.

Because sphinxdocs is almost entirely separate, adding its docs under //sphinxdocs/docs is a more natural fit. Unfortunately, this became very verbose, repetitive, and tedious, for two reasons:

1. The only way sphinx_docs could accept files from other directories was using the rename_srcs arg and manually renaming files one-by-one.
2. Similarly, sphinx_stardocs required a one-by-one mapping of each bzl file to its output file, which then had to be repeated in rename_srcs.

To fix (1), the sphinx_docs.deps attribute and sphinx_docs_library rule are added. The library targets collect files, and sphinx_docs moves then into the final Sphinx sources directory.

To fix (2), the sphinx_stardoc.srcs attribute is added, which accepts bzl_library targets. I noticed that, in almost all cases, the output name was simply the input name with the .md extension, so the rule now does that by default. For special cases, the sphinx_stardoc (singular) rule can be called directly.

Also:

• Adds bzl:rule as a cross reference lookup role
• Removes some defunct stuff relating to the stardoc template files that aren't used anymore.
• Disables warnings from the autosectionlabel extension. These were spamming warnings because CHANGELOG.md has many headers with the same name.
• Adds more entries to bazel inventory (all of native and ctx)

[aignas]

This is super helpful, thanks!