[docs] get current version from pkg_resources
[docs] use current year in copyright
[docs] fix links
remove invalid section from index
[docs] add API docs

There are currently some issues:
* The layout is too narrow
* Everything is on one page
* Sphinx throws several warnings
* The order is random
* Everything is documented
[docs] only document icalendar.cal in the API docs
[docs] indent docstring for sphinx
Merge remote-tracking branch 't-8ch/doc_fixes'
use sphinx_rdt_theme and document how to generate docs
rm old theme
include also prop api doc