docs: tell authors how to publish stardocs

[alexeagle]

Fixes #5593

[gemini-code-assist]

Code Review

This pull request adds documentation explaining how to generate and publish Starlark API documentation (stardocs) for modules in the Bazel Central Registry. The changes include updates to the main README, a new detailed guide in docs/stardoc.md, and a minor schema update to metadata.schema.json. My review focuses on the correctness and clarity of the new documentation, particularly the provided script snippets. I've identified a couple of high-severity issues in the example script within docs/stardoc.md related to inefficiency and inconsistency, and I have provided suggestions to improve them.

Require module maintainers' approval for newly pushed changes.

[meteorcloudy]

Thanks!