This is a documentation change about #15310.

This allows users to add patches as attachments to merge request
created via email.

When an email to create a merge request is sent, all the attachments
ending in `.patch` will be applied to the branch specified in the
subject of the email. If the branch did not exist, it will be created
from the HEAD of the repository.

When the patches could not be applied, the error message will be
replied to the user.

The patches can have a maximum combined size of 2MB for now.

The pngquant tool was used https://pngquant.org, and particularly, the
following command:

        /usr/bin/pngquant -f --skip-if-larger --ext .png --speed 1 image.png

Before: 47584K
After : 34924K

(cherry picked from commit b5ea33397eb8d2c874662026b0e36aed7c41dbbc)

"Maintainer" will be freed to be used for #42751