This commits adds two options to the `toctree` directive:
- raise_level: moves the toctree up in the toc hierarchy
- index: moves the toctree to a specified place among its siblings

Rationale: this gives more flexibility in placing the toctrees in reST
documents and solves the problem with hidden toctrees that are
automatically placed as children of the previous subsection.

This lets the user to change the separator used in generating labels
out of section headings.