[`Docs`] Clarify 4bit docs
# What does this PR do?

As discussed internally with @lewtun , this PR refactors a bit the 4bit docs by adding more clarifications about best practices and giving relevant pointers to users about advanced usage.

Also fixed the requirements instructions as 4bit is now part of the latest release

cc @sgugger 

_The documentation is not available anymore as the PR was closed or merged._