docs rework

Our docs pages are really nice, but IMO, they are currently a bit hard to navigate. That's why I suggest the following changes/enhancements:

• The landing page covers the main topics of the docs clearly using the tiles. The navigation bar on the right hand side however is a bit misleading. I think the titles of the entries there should match the titles of the tiles. Currently we have a few differences: Cookbooks vs. Guides, Functionality Overview (API) vs Anomaly Detection, SaQC Configurator vs. Configurator.
• The first part of the navigation bar on the right hand side should match the tile structure of the landing page and I think that we don't need the separate entries for Anomaly Detection, Data and Flag Tools, Data Manipulation and Data Correction
• I guess we could remove the Repository entry from the side bar menu
• It is somewhat complicated to get a complete list of functions. I only found the way Core -> SaQC. I would have expected to get to this list via the Test Functions entry.