Add CI

This MR does three things:

1. It removes the obsolete Markdown parser
2. It adds the kirsch-handbook commit each page is built from to the footer of each page.
3. It adds CI with automatic deployment to https://sockeye.ethz.ch/kirsch/docs

CI is configured to run on branch main and on merge requests.

Deployment is only done from branch main (i.e. after things were merged).