Move online documentation into CADET repository

[schmoelder]

For discussions, see here.

[schmoelder]

I'm planning on using this tool for the versioning of the documentation. However, for this to work properly, we would have to retrofit the documentation for the older versions (i.e. it only builds for older versions if the folder with the old documentation exists in sphinx format). Or we could just publish newer versions from here on and provide the pdf manuals of the older versions as a download.

[lieres]

I would keep the old PDFs and implement the online documentation from now on. There are more valuable issues to spend time on.

[schmoelder]

I agree. I'd suggest the same for CADET-Match (see here).