Design `Advanced Style Guide`

[lenucksi]

We should develop an initial version of an advanced style guide. This is relevant for the maturity level 4: established proposed by #155.

(Edited by @maxcapraro)

[spier]

@maxcapraro after dropping level 4, we can close this issue here, or not?

[spier]

Closing this issue as we changed the requirements for our maturity levels. Level 4 does not exist any more i.e. an Advanced Style Guide is no longer needed.