docs(gazelle): POC the bare minimum for linking modules_mapping in docs.

[dougthor42]

From #3132 (comment).

While this note exists, the PR description is WIP.

[dougthor42]

sphinx_stardocs doesn't like @rules_python_gazelle_plugin//modules_mapping:def_bzl with:

Error in _stardoc_input_helper:
illegal rule name:
_bzl_api_docs_@rules_python_gazelle_plugin//modules_mapping/def_bzl.primary_bzl_src:
invalid target name '_bzl_api_docs_@rules_python_gazelle_plugin//modules_mapping/def_bzl.primary_bzl_src':
target names may not contain '//' path separators

So I tried the alias thing and it worked 🤷. Having an alias for every bzl_library target is not feasible of course. Maybe a filegroup or a meta-target bzl_library(srcs = ["$ALL_BZL_LIBRARY_TARGETS"])...

With the alias, we are able to link with {bzl:obj}. It goes to: http://localhost:8000/api/rules_python_gazelle_plugin%2B/modules_mapping/def.html#modules_mapping where %2B is +:

rules_python_gazelle_plugin+/modules_mapping/def.html

[aignas]

I think some of the automatic inference withing sphinxdocs needs to be updated, but I am not sure where, I'd need to dig, maybe @rickeylev knows of his hat.