Also change paragraph to subsubsection.

Because the GitLab CI can’t run apt-get at the moment, the font must be
embedded into the project instead of installed in the container.
Also, a switch is added so that if a machine has the font installed, it
uses that version. This is needed because LuaLaTeX doesn’t like using
the project .ttf files on my machine. So this is a workaround for that.

The ubuntu/focal repository provides an old version of the markdown
package which doesn’t support the jekyllData and taskLists options.
Since they are not used at the moment, they are commented out.
Reactivating them would require setting up an additional apt repository.

Moving `.gitignore`, `.bib` and `preamble.tex` files to the root
ensures that they can be used over multiple files.