[Documentation] Describe the main functionality of the tool

[FredericHirschmueller]

So far it is not clear what the tool can be used for. For which "Leistungsphase nach HOAI" can it be used? This must be clearly described within the documentation through the following options:

• YouTube video
• Description in ReadTheDocs
• Workshop
• open_plan website

[c-moeller]

These questions might be related to this issue:

• get a common understanding of the open-plan-tool energy system model (in general, not the details)
• which sectors and main components
• which functionalities and constraints
• which objective function(s)