[ADD] Documentation for OpenUpgrade manager

[legalsylvain]

Hi all.

Rational

I'm currently initializing OpenUpgrade V18.0 branch.
I so follow the wiki documentation (https://github.com/OCA/OpenUpgrade/wiki/)
some remarks :

• there are a lot of pages totally obsolete. (migration from Launchpad ;-))
• there is a diff tools to see the changes of the wiki, but I see several problems.
  -> there are two sources of information. (wiki + Openupgrade/documentation branch)
  -> editing wiki is less user friendly.

Proposal

• I create a dedicated section in the documentation branch, named "Maintaining OpenUpgrade Repository".
• Once merged, we drop the wiki pages, one after the other.
• At the end, we remove the wiki tools to keep only one strong documentation.

This PR

• draft PR for the time being.
• objective : replace https://github.com/OCA/OpenUpgrade/wiki/Set-up-the-branch-for-a-new-Odoo-release

CC : @OCA/openupgrade-maintainers

[pedrobaeza]

OK on the initial approach

[legalsylvain]

PR ready to review. I adapted some points.

Result of the script available here : #4572

Review welcome. Thanks !