Markup: Add rendering of toctree in restructured text

[thomaszbz]

I think it would be of great benefit for readers if the table of contents in restructured text could be rendered. That would allow a preview of project documentation (e.g. for unstable branches) for developers as well as for users trying out unstable software or reading documentation for the next release.

My personal use case would be that I'd like to add a link to the documentation of the project's unstable branch in Readme.md. The project itself contains a sphinx project for documentation.

There's multiple ways to include additional files:

.. toctree::
   :maxdepth: 1

   Introduction1 <introduction1.rst>
   Introduction2.rst
   Introduction3
   Introduction4/Index

An example can be found here: https://github.com/mblaschke/TYPO3-metaseo/blob/develop/Documentation/Index.rst

[westurner]

:+1:

[jonathanunderwood]

This would be immensely helpful

[kivikakk]

We're unlikely to add Sphinx custom directives to our rST implementation.