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

#8417 (closed)
#5542 (closed)
#7368 (closed)

Related issues

Related to #15398 (closed)

Original created by @sajolida on 8416 (Redmine)

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