New `merge4all` program

merge4all is a really bad name, I'm just copying https://about.gitlab.com/handbook/engineering/fix4all here...

The idea is to merge CE into EE and to assign a different developer everyday!

Implementation (take inspiration from https://about.gitlab.com/handbook/engineering/fix4all):

1. Why we need CE->EE merges and how it works
2. Why every developer should be involved
3. Link https://docs.gitlab.com/ce/development/limit_ee_conflicts.html somewhere
4. Link https://docs.gitlab.com/ee/development/ee_features.html somewhere
5. Describe the implementation of this program (link to the CE->EE process: https://gitlab.com/gitlab-org/release-tools/blob/master/doc/merge-ce-into-ee.md)

Related:

• Think of a way to automatize CE->EE merges: #25932 (closed)