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. ← That was the old homepage. Is it available on archive.org?