Port documentation to sphinx

With this MR the Markdown documentation is ported to sphinx (see https://www.sphinx-doc.org).

Basic build instructions can be found here

NOTES

• The MR depends on !10 (merged), which already is under review. Only the last three commits are relevant for this MR.
• The MR includes a small feature commit that also modifies the new documentation. As the feature is small, I decided to keep it here to avoid losing these changes somewhere during conflict solving. The feature is related to CustomCommands as discussed here.