Lots of general documentation is on Trac: http://trac.wildfiregames.com/

Because of a transition (http://trac.wildfiregames.com/ticket/423), code documentation is in two forms:
  http://svn.wildfiregames.com/docs/ (simulation2, scriptinterface)
  http://svn.wildfiregames.com/docs-cppdoc/ (the rest of the engine)

To generate the Doxygen documentation: run "doxygen config" in docs/doxygen/
To generate the CppDoc documentation: run builddoc in build/docs/