Absorbs the Cloudron server-migration material from co-op/runbooks
(now archived) so there is a single public operator documentation
repository. Files move into migration/ (plan, DNS change sheet,
pre-migration state) and provider-pricing/ is kept as its own section;
the merged README replaces both.
History is preserved: this is a merge of the unrelated histories, so
co-op/runbooks' 26 commits remain reachable from this repository and
git log -- migration/ shows their real authors and messages. File
contents are byte-identical to co-op/runbooks (blob hashes unchanged).
