Draft: Feature: Upgrade docusaurus to v3 and add top navbar demo

This branch contains several requested features and examples for them:

• upgrade to Docusaurus v3.1.1, including other libraries to their latest major releases (React, MDX, docusaurus-search, etc.)
• example of Navbar Navigation with dropdowns
• a demo for a React based landingpage, this can be expanded upon by anyone
• an example implementation of how a PDF generator can be used to create a printable version of this documentation

also part of fitko/docs/docusaurus-template#19 (closed)