Support Sphinx ToC generation with _toc_name and _toc_parts attribute

[scottamain]

When generating Sphinx/RST nodes from the Doxygen sources, I'd like Breathe to include the short API member name in the _toc_name and _toc_parts attribute of the desc_signature node. This will enable use of Sphinx table of contents, as originally implemented here: sphinx-doc/sphinx#10807

This attribute would also be available for use from alternative renderers. For example, in a Markdown renderer, I can chose to print the short _toc_name instead of the full signature in a heading. That is, instead of the full signature as a heading like this:

## const char \*M_getError(const [M_Status](types.md#_CPPv48M_Status) \*status)

I could have one like this (using the value from _toc_name):

## M_getError

Thanks for your consideration and all your work on Breathe! ❤️