Expose main TOC to the layout's template

Hello,

one of the current task we're working on, is to provide some preview theme for the user. The theme which Sphinx was providing is [Read the docs](https://sphinx-rtd-theme.readthedocs.io/en/latest/).
Then, in order to build a nice main nav on the left, we would need the parser to expose the environment and / or (?) the main TOC to the template engine.

thanks :)