Get Leo, the outlining editor, and make noweb files. Noweb can make HTML that can be your online documentation and it can make source files that are numbered like your noweb input. You can edit the noweb or the source. If you edit the source, Leo can update your changes back into the noweb file. Noweb can also make TEX files for really pretty printing.
Noweb is so simple its users guide fits on one side of one sheet of paper. Leo is one of those GUI tools you already know how to use ... has lotsa tricks if you want them.
Both are free!