When a project uses fast-forward merging strategy user has
to rebase MRs to target branch before it can be merged.
Now user can do rebase in UI by clicking 'Rebase' button
instead of doing rebase locally.

This feature was already present in EE, this is only backport
of the feature to CE. Couple of changes:
* removed rebase license check
* renamed migration (changed timestamp)

Closes #40301

* fixed anchor for create MR from email
* added a screenshot for this feature

When a merge request is created from email, use message body
as merge request description. If message body is empty then
merge request description is still created from the source
branch commit (if there is only single commit in the merge
request).

If message body is empty and there are multiple commits in
the source branch, then merge request description is left empty.

Closes #40968

* new merge request can be created by sending an email to the specific
email address (similar to creating issues by email)
* for the first iteration, source branch must be specified in the mail
subject, other merge request parameters can not be set yet
* user should enable "Receive notifications about your own activity" in
user settings to receive a notification about created merge request

Part of #32878

Already mentioned in the list above this one.

from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2350
+ copyedit (goes  with 80edf003)

- Move to dicsussions/index.md
- Bring back previous path with a link to new location
- Reorder sections

By using `>`-quote style instead of `>>>`.

Also: Update documentation images.