chore(docs): document module extensions

[alexeagle]

These are the primary API under bzlmod

Also pull out the docgen snippet so we can call it from a different pipeline, like I did in rules_ts and rules_js

[aspect-workflows]

Bazel 7 (Test)

All tests were cache hits

1 test (100.0%) was fully cached saving 50ms.

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Bazel 8 (Test)

All tests were cache hits

1 test (100.0%) was fully cached saving 50ms.

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Bazel 9 (Test)

All tests were cache hits

1 test (100.0%) was fully cached saving 35ms.