Change documentation structure

[SebastianGnann]

For the online docs I created HTML files of all signatures etc. using m2html (https://github.com/gllmflndn/m2html). This works great but is not automatically updated. This feature is nice but possibly not necessary, as all the code can be viewed on Github as well (which always shows the most recent version). So perhaps it would make sense to change the documentation accordingly.