Documentation Structural Refactoring

After going through the docs, I'd like to take on refactoring the structure of the docs, and adding some missing pages.

First, it probably makes sense to gather all the "types" of pages / doc information that we want to display, and then to see the structure. This should be an exhaustive list of pages, so please comment and I'll edit the missing bits in the issue description:

• Introduction
  • Concepts (new) (an overview over the types of components and concepts, maybe mindset behind how StackGres works)
• Features (an exhaustive list over all features that StackGres provides, with reasoning and individual "deep dive" explanations & how-to's)
  • Feature "deep dive" (include explanations of what the individual features can do, with the assumption that the reader has read the concepts/architecture page, and self-sufficient examples of how to configure an SG cluster to do X)
• Getting Started (a quick start how to get to a running StackGres, i.e. to try it out quickly)
• Administration Manual (to document specific common tasks, i.e. slightly more complex scenarios than described in the feature deep dive)
  • Production Installation (refactor duplicate content from rest of Admin Manual)
• CRD Reference
• Developer Documentation
  • Operator API
• Runbooks
• FAQ (new, content TBD)

(this is sort of blocked by #2220, I'm still creating the issue already for our reference)

All of this is up for discussion, please comment and I'll update the issue description accordingly.