Adding a new helm chart to deploy functional mail server on Kubernetes.
It's composed of postfix and dovecot so far and handles user management
using Kubernetes secrets.

This patch adds some external links to the docs navigation section.
Since mdbooks doesn't support this by default, this adds a JS file as
workaround, which avoids forking the theme or writing an extension for
mdbooks.

References:
https://github.com/rust-lang/mdBook/issues/1354

This patch adds a title to the documentation as well as some links to
the repository as well as the WebIDE for GitLab adjustments.

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