[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 :

• Maybe the Rebase your branch title is too big ?

[stof]

adding a heading here breaks the steps.

the case of rebasing should be a section later in the doc, not in the quick start guide for your first contribution (and maybe it could link to the documentation about rebasing a PR for code contribution instead of duplicating it, as it works exactly the same, given that this is purely about git usage)

[AndoniLarz]

I agree with the idea of just using a link and made a commit accordingly.
However, I believe the link would be useful here so that beginners don't feel lost if they face this case. Be it because they began working days or weeks ago, or just minutes ago and by chance a PR got merged just in the meantime.