Restructure Cabal documentation top-level parts

The goal is for users to easier find pages for typical problems through search engines and page navigation.
- The top-level layout is based on the popular documentation structure by https://documentation.divio.com/ to give a
   clear structure to users and future documentation contributors:
  * Guides: Present a solution to a single, atomic, typical user problem.
  * Reference: Describe user API (CLI fields, syntax etc) with technical rigour and completeness.
  * Explanation: Discuss background information, scope, design decisions etc.
- Move existing documentation roughly into these categories with minimal editing as the basis for further editing.
- Rename guide titles to mention how-to for improving SEO.
- Rename some files to improve SEO since that name becomes part of the URL (often called slug).
  Important page keywords should appear in the slug as well to make pages rank higher in search engines.