Add sections to flax.linen toctree

[cgarciae]

What does this PR do?

Fixes #3069

• Splits flax.linen.rst into multiple sections to make the table of content easier to navigate.
• Creates a new flax_module directive which uses the flax_module.rst template to be able to show the Module subclasses in the same page.
• Relocates all Module implementations under the Layers category
• Merges nowrap and compact into the Decoratros category
• Renames RNN primitives to Recurrent

Preview

Live: https://flax--3073.org.readthedocs.build/en/3073/

[codecov-commenter]

Codecov Report

Merging #3073 (ca86757) into main (4f9f64e) will not change coverage.
The diff coverage is n/a.

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  Coverage   82.19%   82.19%           
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  Files          54       54           
  Lines        6016     6016           
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  Hits         4945     4945           
  Misses       1071     1071           

[chiamp]

This is great, thanks for setting this up! Some suggestions:

Consider adding Module methods on the right hand side of the Module page, e.g.:

On the Activation functions page, there is a discrepancy between the left and right hand side list:


Can we remove param_dtype and negative_slope_init as they aren't activation functions?

[cgarciae]

@chiamp I tried to fix the PReLU, however, I don't think we can add the methods from Module to the right hand TOC. Currently it doesn't behave like you suggest either. We could leave it as an issue for the future.