If all you want to add is a key combo to toggle fullscreen you can go in to the file "4coder_generated\remapping.h" and add a line like this (or using other keys, remember to disable or change those bindings that only use the PageUp key if you use this combo)
key_page_up being the PageUp key and
MDFT_CTRL being the Ctrl key and
toggle_fullscreen begin the function that would be called.
bind(context, key_page_up, MDFR_CTRL, toggle_fullscreen);
Then after that you run buildsuper and restart 4ed.exe
There are some video tutorials on Allen's Youtube channel but other than that and http://4coder.net/custom_docs.html
I don't know of much more documentation, personally I learned most of how to change and what to change by just reading the code and watching some of his streams, and making simple customizations starting with copies of Allen's functions from the custom layer.