Document MAT and metadata
Our main documentation index lacks a page about metadata and MAT.
It should also mention pdf-redact-tools, tesseract-ocr, and ffmpeg. See #17178 (closed).
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.
- Related to #15398 (closed)