Document MAT
Our main documentation index lacks a page about metadata and MAT.
The homepage of MAT (https://mat.boum.org/) is pretty good at introducing what metadata are. So we should definitely point to it, and maybe reuse parts of it.
Blueprint: https://tails.boum.org/blueprint/mat/
Subtasks
Related issues
- Related to #15398 (closed)
Original created by @sajolida on 8416 (Redmine)