general documentation files reside in the doc/ directory, where this readme file lies.
you can find ideas, milestones, planning and workflow descriptions etc in here.
a good entry point may be the file doc/implementation/project_structure as it roughly explains where to find what code in the project.
if you made any relevant discoveries, create a file and write them down.
dynamic documentation files are generated from comments in the source code, using doxygen. the dynamic docs tend to only describe stuff you could also understand by reading the code itself, so don't expect too much, use the static docs instead.
after you configured the project, you can invoke
make doc
to create doxygen html and LaTeX files.
after creation, view them in a browser by
$(browser) bin/doc/html/index.html
or, if you want to create LaTeX documents:
make -C bin/doc/latex/ pdf
$(pdfviewer) bin/doc/latex/refman.pdf