feat: publish stardocs on releases, rather than write_source_files

[alexeagle]

The current workflow for docgen is awkward: contributors who change a .bzl file get a red PR telling them to also update a generated .md file in the repo.
It has an ugly dependency shape as well: running stardoc requires a transitive Java and Maven dependency. While bzlmod makes this transparent in the repo's sources, we still observe that slowness when running the target. This means a casual contributor who makes a spelling fix in a .bzl file is immediately confronted with having to perform a long, slow build to get their PR green. Not a good experience for someone evaluating whether they want to use Bazel!

This workflow is replaced by:

• use bazel_lib's bzl_library to fix bzl_library does not validate the inputs are correct bazelbuild/bazel-skylib#568
• as a consequence, we can query for native starlark_doc_extract rules and request their outputs
• feat: render stardocs bcr-ui#198 locates the doc_urls from source.json and renders on BCR, for example scroll down on https://bazel-contrib.github.io/bcr-ui-preview/pr-preview/pr-198/modules/tar.bzl

Note, I've already made this change in several downstream repos, such as tar.bzl and bazel-lib itself, so the pattern is established.