Make `adiar::internal` its own Module in Documentation

[SSoelvsten]

To help developers, we should allow the documentation to be compiled with internal stuff too.

• Add ADIAR_DOCS_INTERNAL as a CMake option
  • Add docs/internal for a definition of the Internal module and its subgroups.
  • Add files to Doxygen INPUTS (see docs/doxyfile.in)
  • Make each header file in <adiar/internal/...> start out with adding itself to the Doxygen module module__internal.
  • Hide the entire internal module if ADIAR_DOCS_INTERNAL is OFF.
• Add ADIAR_DOCS_TODO as a CMake option
  • Convert relevant TODO: into \todo

Additional Context

Requested by Erik Funder Carstensen while working on an MSc course project.

[SSoelvsten]

Seeing the first draft for the Internal module on the docs/internal branch, it needs much more work. Right now, it just follows the folder structure. But, doing so still leaves every folder as a giant mess of disparate definitions. Especially the internal/algorithms folder is completely impossible to browse.