Adds standardize initializer

[marcvanzee]

What does this PR do?

Fixes #2119 (issue)

Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other
  checks if that's the case).
• This change is discussed in a Github issue/
  discussion (please add a
  link).
• The documentation and docstrings adhere to the
  documentation guidelines.
• This change includes necessary high-coverage tests.
  (No quality testing = no merge!)

[cgarciae]

@marcvanzee please add the entry for standardize in flax.linen.rst 🙏

[cgarciae]

@marcvanzee thanks for adding all the missing functions, looks great!

[marcvanzee]

It seems our RTD was quite outdated for activations (some were missing) and initializers (none were there), so I added all of them.