Discussion: Update Coding Guidelines

[reneSchm]

Provide a description of requested documentation changes.

We need to discuss and update some guidelines. For example, they still claim that we use C++14, when we've been on C++17 for a while now. Also, it is unclear how to do include guards, as they are used inconsistently even within directories. Some files adhere to the guidelines, others use MIO_<filename> or MIO_<dir/namespace>_<filename>.