Documentation should contain a "Troubleshooting" section

Standard/common problems and instruction to help user debug their problems should be documented in something like a "troubleshooting" section.
It should probably include:
- checking that the user is in the appropriate VO/COU
- mention that there may be a need to link various identities or to ensure that it's always the same IdP that is used
- how to try to debug (using standard CLI/alternative tools)
- who to contact