[Contributing] Add documentation for rebasing when contributing to the docs

[AndoniLarz]

Title pretty much says it all : I asked on Slack about how to rebase my docs branch because I'm not used to rebasing.

We noticed there is no such documentation on the Contributing to the Documentation page, whereas there is one in the Proposing a Change.

So here is the PR to add such a section.

QUESTIONS :

• [x] Maybe the Rebase your branch title is too big ?