Improve documentation based on TUDelft and CWI review

[FrankKr]

[X] Add os.chdir('.') [X] Also store the code in the html [X] Make seperate Methodology page on openstef

• Add complete overview of features. And add description of what they mean.
• Add overview of methodology; what does the input and output look like. How do we use 1 model for multiple timestep prediction

[FrankKr]

Done!:)

[FrankKr]

Result: https://openstef.github.io/openstef/concepts.html#high-level-methodology-openstef