We use doxygen for source code and API documentation. It is integrated into Trac, just click the Doxygen button in the menu above.

In addition to doxygen there is a separate section of the Wiki for longer, conceptual descriptions and explanations as well as tutorials.

They are ordered and put in a sensible order on wiki:Doc/DevGuideFull, but you can also see the individual pages:

DevGuide/Logging