docs: Initial documentation set up

This patch adds the initial structure for the documentation of the
Shivering-Isles GitOps Infrastructure repository. It uses mdbook to
create an easy to read documentation.

It uses mdbook, because it's an easy single-binary tool to generate
documentation. This fits the tooling schema of koolbox quite well and
can be easily integrated into the automatic updates toolchain already
used. Further it uses markdown, which allows easy writing of further
documentation and re-use of existing documentation.

The documentation structure re-uses various existing READMEs from
components. These might be replaced in the long run.

It also lays out future plans for documentation, which is currently
still on the TODO list. This should help to keep the overall structure
in mind and provide the general direction.

References:
https://rust-lang.github.io/mdBook/index.html