Document restoring core and coding system release DBs from droplet backup

[iaindillingham]

Document the process of restoring the coding system release DBs from a droplet backup. Doing so will mean actually creating a new droplet, and restoring a droplet backup into it, to verify that the process works. #2116 defines the terms coding system release DBs, droplet backup, and droplet.

[lucyb]

I'm closing this as I believe it will be made redundant by #2225 and a move over to Digital Ocean Block Storage.

[mikerkelly]

This was reopened as if we rely on the droplet storage as a fallback restore option as mentioned in #2246 it would be beneficial to verify that we can actually do that and understand what it would involve.