Write https://tails.boum.org/gdm

Originally created by @sajolida on #15399 (Redmine)

Pointed to from config/chroot_local-includes/lib/systemd/system/tails-gdm-failed-to-start.service.

Feature Branch: doc/15399-gdm-debugging

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