[Feature]: Add documentation to module metadata

[RyanDraves]

Allow modules to provide documentation visible in its entry in the BCR. This feature would be similar to how a Markdown file can be submitted alongside an upload to PyPI, however these docs would have the additional PR review PyPI uploads lack.

Maybe repos already dedicate a simple Markdown file to important usage details, for example pico-sdk.

The inspiration for this change was using the BCR entry for cppzmq. In this scenario, Bazel wrappers not present in the source repo are added to this repo, with usage information (one must also link against the libzmq module) only present in the PR description. With this feature, the Bazel module maintainers could provide documentation separate from the original repo.