Section nesting

[GB609]

It should be possible to nest sections.

For this, the level of the section header and function_nesting should instead be calculated:

1. section_nesting starting at 0
2. when @section is encountered do
   • section_nesting += 1
   • function_nesting = section_nesting + 1
3. on @endsection
   • section_nesting -= 1
   • function_nesting = section_nesting + 1
4. render_docblock when called from within process_section should pass section_nesting

This is a behaviour change.
Currently, anytime @section is encountered, the nesting levels are hard-set to section: 2, functions: 3.
This means, that @endsection isn't required to remain on the same nesting level, any new section will start fresh.

When this change is implemented, @endsection is basically required to achieve the same because shdoc alone can not determine if further nesting of @section is desired or not.

Not using @endsection would continue to increase nesting levels when encountering @section until a depth is achieved for which shdoc does not have code templates anymore.