CI: split documentation publish on [master_branch]

What

As on Before_merging, we split the documentation build.

Why

The documentation:publish is getting too slow, sometimes even hitting the one hour timeout.

How

Manually testing the MR

• Made a fake master_branch here: https://gitlab.com/tezos/tezos/-/pipelines/1452348460
  • I have not checked that the docs are exactly as before, but the main things seems to be there ;)

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR