Support module docstrings

Code change made in https://github.com/bazelbuild/bazel/commit/5faf9525ff1a9f3127139c8f5f5db3e2eb1481fe to support adding module docstrings to Stardoc output. The templates still need to be updated to reflect this new feature.

Start by reviewing the referenced Bazel commit and the Stardoc templates it changed or enabled. Compare current Stardoc output with the module-docstring support described in the issue; done means the templates represent module docstrings in generated documentation.

Written by the indexing model from the issue text.