devicetree: improve Doxygen docstrings

- Use `@code`/`@endcode` commands so that examples will have code
  highlighing
- Remove redundant `@ref` commands
- Fix some incorrect `@p` usages (it is for function/macro parameters)
- Use `@p` name to reference macro arguments
- Convert a few "quoted strings" to literals using Markdown syntax (``)
- Always use Markdown syntax for literals (no `<tt>`, `@c`, etc.)
- Move implementation details to `@internal`/`@endinternal` blocks inside
  the relevant macro.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>