Create skeleton for doxygen comments

[akorgor]

This commit creates a skeleton for Doxygen comments. It highlights locations where doxygen comments should appear with simple placeholder comments, most of which need to be improved. This PR is meant as a discussion base for agreeing on an appropriate content level and layout of these comments and is thus a work in progress.

Currently, all comments are one-liners and thus follow the \\! syntax. However, multi-line comments can be created with

\**
  * @brief Comment
  *
  * More details here.
  * 
  * @param parameter
  * @returns returned variable
  * \

Every function with the override keyword inherits the virtual function's doxygen comment and thus does not need its own docstring unless there are mention-worthy differences.