doc: Overhaul doc guidelines

- Improved document structure (was *very* flat before) by adding more
  meaningful top-level headings and re-ordering some of the existing
  sections
- Provide detailed and structured docs for all the custom Sphinx roles
  and directives available in Zephyr, including moving the docs for
  ``.. zephyr-app-commands`` directive and `:dtcompatible:` roles away
  from the extensions' source files to align with how Sphinx typically
  handles docs for custom roles/directives.
- Added more content regarding diagrams guidelines, images,
  cross-referencing of Doxygen documentation, ...

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>