make (or document) a "gitlab maintenance mode"

in #41431 (closed), we found out we couldn't set a "maintenance mode" in gitlab. that would have been useful to do operations on the VM (@lelutin: which exactly?) to improve the VM transfer.

i can also imagine this being useful in other cases.

i wonder if simply stopping gitlab would be sufficient: then Nginx is the one responding with a 500 message. for the VM migration, that doesn't work though because the whole VM is down. then I wonder if the right way wouldn't have been to simply setup a static site for VM and point DNS there while the transfer happens.

either way, we should make a procedure for this, documented in the gitlab service docs.