Documentation: revise configuration section

[confluence]

This section should be expanded to explain different possible configuration options in more detail. Features should be versioned.

Related issues which should probably be fixed here:

• OpenID documentation #132
• Add a changelog #203
• Documentation: add a "troubleshooting" section #204
• Documentation: remove version from prefix example in configuration section #212

Related issues which need to be fixed in the schema and/or code:

• Default kill script location in schema #205
• Feature suggestion: command to generate default configuration #206
• Problem with defaults not being handled correctly across multiple configuration files #207
• Normalise and merge schemas; make other components use schemas from this submodule. schemas#1
• Parse config files as JSONC carta-backend#1404