Add docs on sphinx tags

Another Sphinx feature that would be helpful would be addition of Sphinx tags:
http://www.sphinx-doc.org/en/stable/markup/misc.html#including-content-based-on-tags

It allows for having content like:

```rst
.. only:: footag

    Only in the Foo docs!

.. only:: bartag

    Only in the Bar docs!
```

The tags config option is already exposed in conf.py