Moved pooling OP from linen to core, added pooling documentation to NNX

[jorisSchaller]

This PR

• Moves the pooling operation from flax/linen/pooling.py to flax/pooling.py
• Adds the missing pooling documentation tonnx
• Updates the linen code to import pooling from flax/pooling.py
• Update the nnx code to import pooling from flax/pooling.py

Fixes #4271

Checklist

• [x] This change is discussed in a Github issue #4271

• [x] The documentation and docstrings adhere to the documentation guidelines.

• [] This change includes necessary high-coverage tests. (No quality testing = no merge!)

Hey, So the changes should be straightforward, nonetheless, I had trouble running all the test so i'll wait for the CI/CD to tell me more.

EDIT: Edited the path according to https://github.com/google/flax/pull/4408#discussion_r1863344668

[cgarciae]

Thanks for doing this @jorisSchaller! Left a comment. Looks good.