explain what optax.centralize does

[fabianp]

Its docstring is extremely poor: https://optax.readthedocs.io/en/latest/api/transformations.html#optax.centralize

• Explain what centralize means
• Add a usage example
• Put reference in correct RST format

[miguelcsx]

Hi @fabianp , I'm going to start working on this issue, could you assign it to me?

[fabianp]

thanks for picking this up @miguelcsx !