Draft: Updates for the Repository Mirroring Documentation

What does this MR do?

As I free user I noticed I can:

So this MR updates the Repository Mirroring documentation to better reflect current FREE plan features on the Repository Mirroring feature documentation.

I moved the Mirror only protected branches up in the tree so the premium features comes above it to make more sense.

Can I get the confirmation in which version of Gitlab theses features got implemented in the free plan so I can update the history before the merge?

Does this MR meet the acceptance criteria?

Conformity

• I have added/updated documentation, or it's not needed. (Is documentation required?)
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.