Separate the TOC sections

[LecrisUT]

Hopefully this helps with navigability a bit

[gemini-code-assist]

Code Review

This pull request refactors the main table of contents in the documentation into separate, captioned sections to improve navigability. A syntax error in one of the new links needs to be corrected.

[happz]

Why not, although I'd wait for few more approvals, the actual grouping might be a point of discussion.

[psss]

Copy pasting the structure brainstorm from #4704:

• How it works under the hood
  • The basic split/flow of user tree + (user config) -> run -> step -> phase -> test
  • The various trees and directories
  • The split between guest
  • Plugins
  • (Recipe eventually)
  • Glossary
  • Advance topics:
    • Phases that create other phases
    • Profiles/Policies
    • Phases that jump across steps, e.g. dist-git discover and upgrade
• What feature is available right now/How to use tmt
  • Specification/Features what you can do: multi-host, restart, hardware, etc.
  • External plugins
  • Examples and guides
  • Projects that use tmt and their feedback
• Code details
  • Python API
  • Config reference/index