Document machine deletion process for tarsnap backups

[Krinkle]

TODO at https://github.com/jquery/infrastructure-puppet/blob/staging/doc/backup.md

[Krinkle]

As part of https://github.com/jquery/infrastructure-puppet/issues/12, I did the following for cla-01.ops.jquery.net:

$ sudo tarsnap --list-archives
2023-07-01-040001-monthly-cla-output
…
…
…

Then pass each to sudo tarsnap -d -f <name> like sudo tarsnap -d -f 2023-07-01-040001-monthly-cla-output.

Or to delete them all:

sudo tarsnap --list-archives | xargs -P 1 -n1 sudo tarsnap -d -f

[Krinkle]

• Ref https://github.com/jquery/infrastructure-puppet/commit/2299caeb9034aea138716989f8dda0b775574499.
• Ref https://github.com/jquery/infrastructure-puppet/commit/8a57d01e9da1167fa71bb8cabe55c95ffa9db784.