Add documentation about troubleshooting

[maxidorius]

Add a new document that gives basic information about troubleshooting like:

• Where to find the logs and how to read them
• Common errors and how to recognize them in the logs

This is closely related to #102 but should be aimed at how to recognize problems across all features and all built-in Identity Stores rather than find out if a basic setup is working. Given the scope of the issue, this will be broken down into smaller tasks later on.

[maxidorius]

Done as part of the v1.4.0 release.