Restructure guidebook content

The DevOps guidebook, while fairly complete and comprehensive, grew organically and is thus not structured in a way which encourages visits.

The Diátaxis framework discusses good practice for documentation and identifies that generally people come to documentation for one of four reasons.

Re-structure the guidebook around the following sections:

• Tutorials
• How-to guides
• Reference
• Explanation
• Overview
  • ... for external users, containing the current contact us pages.
  • ... for internal users, describing the documentation structure.