resourceloader: Add Doxygen group and improve overall docs
* Add license header where missing.
* Add missing `@since` (1.17 for most classes), except
ResourceLoaderLessVarFileModule since 1.32 (
1bc62c548c).
* Remove duplicate file-level description for class-only files,
merge with the class description instead.
* Remove my own `@author` annotation from one file.
* Mark core's own FileModule subclasses as `@internal`, except
for the following which we support use of in extensions:
ResourceLoaderLessVarFileModule,
ResourceLoaderOOUIIconPackModule, and
ResourceLoaderWikiModule.
Change-Id: I336af2e4ccdbe2512594e8861b72628d24194e41