I haven’t read your post in full to attempt an answer at your specific question, but this workshop was made exactly for people wanting to write documentation with documenter.jl , even starting just from pure scripts or source code functions :
an answer should be there.
As for your unsolicited opinion 
: it is entirely possible to use Documenter.jl locally without knowning much, if anything, about the concept of modules and packages. However, doing so will make everything so much harder for you. It is literally easier to first learn the basisc of packages (5 minutes max in the official docs) and then go to documenter. And the advantage: you also learn about environments as you learn the other things, which a very important component of reproducible workflows.