Add notice about DebOps examples to "getting started" page.
Change some headings in "getting started" page.
Add project description to documentation main page.

This text is taken from the projects home-page.
Add contributions page.

This page was shamelessly taken from
<https://github.com/debops/debops/blob/master/CONTRIBUTING.rst>
Add documentation for debops-defaults.
Fix `Unknown target name error`.
Merge pull request #25 from htgoebel/master

Add contributions page and project description to documentation main page.