Split out `//sphinxdocs` to be a separate bazel submodule and setup publishing to BCR

[aignas]

It seems that having a non-dev dependency on stardoc means that rules_python is pulling in all of the transitive dependencies of stardoc and we may get rules_kotlin in the closure. That is sub-optimal and could be improved, but needs help from someone willing to dedicate the time.

List of things that would need to be done:

• Add MODULE.bazel to //sphinxdocs folder.
• Move the management of sphinx related PyPI deps to that MODULE.bazel. Currently dev_pip hub repo has more than is needed by the docs only, I think.
• Setting up the BCR bot publishing similar to how gazelle is setup.
• Add a note to the CHANGELOG for the users on how to migrate.
• Provide WORKSPACE setup (sphinxdocs_deps.bzl and sphinxdocs_setup.bzl

Out of scope:

• Clean solution to the sphinx_bzl Python library and its dependencies. This isn't solved today, so no need to block splitting out a separate module on this problem.

Context: #2498