Our documentation on management commands was very limited, listing only

a few commands and in no specific order.
This change completely reworks the documentation to list all common

commands we display in rb-site manage --help. They go over basic

usage and options, show warnings for the more dangerous commands, and

provide links to useful reference docs.
Each set of commands are listed in a dedicated section, providing

organization.
There's a new rb-management-command directive and role, which will

make it easier to link to the necessary docs without using :ref:.
Two commands, find-large-diffs and invalidate-api-tokens, have been

added to the mange rb-site manage --help output.

Built the docs. Checked for build errors, spelling errors, and bad

links. Viewed them in the new docs UI.