Document mirror - how to create and deploy a mirror from scratch

As part of the January 2022 documentation sprint. Create documentation for mirror, here is a guided HOWTO for institutions and organizations that want to have a mirror on their side.

List of documentation elements needed for the HOWTO:

Migrated from T3829 (view on Phabricator)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information