Document Git repo of ISO archive

We have a private repo with the archive of ISO images.
It was made private to avoid having to deal with the impact on our
server resources (e.g. bandwidth) and associated costs.

But we can, and do, create accounts for contributors who need access
to that archive: ask tails-sysadmins@boum.org.

Feature Branch: doc/11795-iso-archive

Related issues

Related to #12626 (closed)

Original created by @sajolida on 11795 (Redmine)

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