docs: Add module reference

docs: Add module reference

None of the modules and functions in toolbox are documented. Thus,
users have to read the source code to understand how functions work,
and to find out what functions are there to begin with.

This commit adds a module reference, documenting modules and the
functions that they implement. The following modules are documented
in the reference:

 - array
 - conf
 - mutex
 - wmutex
 - git
 - inst
 - queue
 - opt
 - is
 - sem
 - json
 - log