There have been several requests for FreePBX/Asterisk docs.
The approach we took was to create a series of “cheat sheets” each of which addressed a specific feature or issue. Since our system was deployed in city government, we also devised a “cheat sheet” specific to a a particular department.
I’ve had several requests for what we did, so I’ve made them available for download.
Hopefully they will give you some ideas. The only thing I ask is that if you develop something based on these or something similar you share them with the FreePBX community.