Improve documentation generation instructions in CONTRIBUTING.md

[Ayoub-Mabrouk]

CONTRIBUTING.md explains how to generate docs, but not when you need to do it or that you must commit the changes. This causes CI failures when contributors don't realize they need to regenerate and commit the docs.

I'd like to:

1. Add a composer docs command to make it easier
2. Update CONTRIBUTING.md to clearly explain when/why you need to regenerate and commit

This should help prevent the CI failures.