Replace wiki pages on github by a web page (ghpages) and readthedocs

[mortenwh]

This page (https://github.com/nansencenter/nansat/wiki/How-to-work-with-github:-scenarios) is not updated with our conventions...

[aleksandervines]

Something weird is going on, as Anton was not aware that he kept reopening issues.

[mortenwh]

forgot to update build documentation when adding these http://nansat.readthedocs.io/en/develop/source/documenting.html