Add test of documentation coverage

[jakevdp]

A frequent documentation issue is that we add public APIs without adding them to the appropriate place in the docs. This is a new test that will check this automatically. There are two main pieces:

• a function which crawls the docs directory looking for documentation of various functions
• a function which crawls the jax package tree looking for public APIs

Finally, there is a test that compares these two and errors in the case of undocumented APIs. This script also allows for specification of submodules which should be skipped entirely (to avoid e.g. ImportErrors in cases where optional dependencies are required), as well as particular APIs that are expected to be undocumented.

The test reveals many potential fixes; I will plan to patch-up the documentation as a followup.

[gemini-code-assist]

Summary of Changes

Hello @jakevdp, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a crucial new test designed to automatically enforce documentation coverage for JAX's public API surface. By programmatically identifying both documented and public APIs, this test will help prevent future regressions where new public functions or classes are added without corresponding documentation, ensuring a more complete and up-to-date user guide.

Highlights

• New Documentation Coverage Test: A new test (documentation_coverage_test.py) has been added to automatically verify that all public JAX APIs are documented.
• API Discovery Mechanism: The test includes functions to crawl the docs directory for documented APIs and the jax package for public APIs, then compares them.
• Configurable Skips: The test allows for specifying submodules to skip entirely and individual APIs that are intentionally undocumented.
• Documentation Updates: Minor additions were made to jax.nn.initializers.rst and jax.nn.rst to include previously missing API entries, and a docstring was added to the Initializer protocol.

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[gemini-code-assist]

Code Review

This pull request introduces a new test to ensure documentation coverage for public APIs, which is a great addition for maintaining the project's quality. The implementation is solid, using file parsing and module introspection to compare public APIs against documented ones. I've made a couple of suggestions to improve the new test file's maintainability and correctness. The documentation updates in other files look good.