Not familiar with any docs, neither found on docs.mendix.com, but yout can start here:

and get to know the structure:

and their purpose:

.git - Since Mx10 the version management (used to be svn)

.mendix-cache, well, contains the cache. Will autocreate again if you manually delete it

deployment - Contains the files that get created upon application start.

GPUCache is used to improve the performance of graphical rendering by offloading some of the processing tasks from the CPU (Central Processing Unit) to the GPU

javascriptsource, where you will find subdirectory per module, and if that module contains any javascript: then also a directory action, containing the javascript

javasource, where you will find subdirectory per module, and if that module contains any java: then also a directory action, containing the javacode

mlsources is part of the infrastructure that supports the integration of machine learning models into Mendix applications

resources containing any image, text or other custom file that you want stored in the application-code

theme contains the general theme docs

theme-cache any cache-files, Will autocreate again if you manually delete it

themesource contains the module-specific theme docs

userlib contains the java's jar-files of the modules you imported

vendor-lib contains the jar-files of third-party modules

widget contains the java's jar-files of the widgets you imported