Add Documentation

[Firionus]

Some suggested content:

• API Docs
• Document return values of functions. Follow this template: https://discourse.julialang.org/t/coordinating-community-efforts-to-enrich-function-docs/80338/45
• There should be a diagram and concise explanation of the resampling approach in the README.
  • Create Diagram Code
  • Set Up Documenter and README Documentation
  • Is Doctest run?
  • Document an Example in the docstring where all args are used
  • Document how to construct and call your own window functions
• Diagrams and Explanations of the different windows and their parameters

Examples, e.g. frequency response measurements -> do that in smoothing library