Modernize user and repository "delete" workflows and improve documentation

Summary:
Fixes T8830. Fixes T13364.

• The inability to destroy objects from the web UI is intentional. Make this clear in the messaging, which is somewhat out of date and partly reflects an earlier era when things could be destroyed.
• bin/remove destroy can't rewind time. Document expectations around the "put the cat back in the bag" use case.

Test Plan: Read documentation, clicked through both workflows.

Maniphest Tasks: T13364, T8830

Differential Revision: https://secure.phabricator.com/D20694