doc/cephfs: repairing inaccessible FSes

doc/cephfs: repairing inaccessible FSes

Add a procedure to doc/cephfs/troubleshooting.rst that explains how to
restore access to FileSystems that became inaccessible after
post-Nautilus upgrades. The procedure included here was written by Harry
G Coin, and merely lightly edited by me. I include him here as a
"co-author", but it should be noted that he did the heavy lifting on
this.

See the email thread here for more context:
https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/message/HS5FD3QFR77NAKJ43M2T5ZC25UYXFLNW/

Co-authored-by: Harry G Coin <hgcoin@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>