Umbrella PR: Content and index page restructuring

[martin-helmich]

This PR implements a major restructuring of both the index page and the content structure itself.

In detail:

• The OpenAPI reference is now built using a set of custom React components (and a few glue scripts); the Redocusaurus-based reference was dropped and appropriate redirects were added where necessary.
• The content structure now consists of the two major sections "Platform" (formerly "Technologies") and
  "API". "Technologies" was renamed to "Platform", because it is more precise and can be used as an umbrella
  category for all kinds of hosting platform features, tutorials and how-tos.
• The index page was restructured to mirror these two sections, and also contains deep links to the most
  important features.
• Redirects were added whereever URLs were changed
• Added documentation for Python apps and extracted common documentation parts from the Node.js
  documentation