Add documentation

[Daafip]

• [x] example notebook
• [x] make docs with GitHub pages?
  • edit: now read the docs: https://ewatercycle-hbv.readthedocs.io/en/latest/index.html
• [x] add API reference to docs
• [x] #38

[Daafip]

pull request #20 adds the beginning of docs

[Daafip]

live at: https://ewatercycle-hbv.readthedocs.io/en/latest/index.html Moved last bullet to new issue: improvement