website improvement

[bniebuhr]

To do:

1. add information about the functions in the main page
2. document the arguments and output of the functions within the code
3. create a page for the documentation of each function, exaplaining its purpose and each argument
4. add spatis calculation page (appendix S4)

[bniebuhr]

5. solve the issue with colors of the navigation bat
6. add numbers to the TOC of the subpages