[TASK] Migration documentation

Moved documentation to reStructuredText

Resolves: #57621
Releases: 2.4
Change-Id: I178879d667badcf340fff5dc3cc27b9a9120ee39
Reviewed-on: https://review.typo3.org/29196
Reviewed-by: Francois Suter
Tested-by: Francois Suter