document ud-ldap and its architecture better

our LDAP documentation is minimal. go through the thing and document how the different components play with each other and common tasks (like creating a user and so on).

i'll consider this complete when i have:

• pager playbook: https://gitlab.torproject.org/tpo/tpa/team/-/wikis/howto/ldap#pager-playbook
• disaster recovery
• install instructions? (there are none!)
• design analysis, started in https://gitlab.torproject.org/tpo/tpa/team/-/wikis/howto/ldap#design
  • a good walk-through of ud-generate (esp. including all the exportOptions possible and generated config files)
  • ... and ud-mailgate
  • an overview architecture summary...
  • with a neat diagram of the moving parts
  • expand based on this audit docs
  • web interface
• monitoring and testing section
• logs and metrics section
• a discussion of problems ...
• ...and possible solutions with the current design
• better user docs, maybe moved out of db.tpo? for example: "how to change my email forward" (#40059 (closed)) is undocumented - this requires updating the packages first, as upstream has some improvements we do not have, see #40062