Provide tutorial requirement information

[josh146]

The demonstrations page should have a section at the bottom, providing the Python environment details used to build the tutorials.

For example, something like the following:

.. rubric:: Running the tutorials

    All tutorials are built using the following requirements.

We could either include directly the text within requirements.txt (will probably look ugly), or just link to the file on GitHub.

[josh146]

Addendum: this is not entirely accurate.

While all executed demos (those with filenames beginning tutorial_) will share the same requirements as the top level requirements.txt file), non-executed demos have requirements known only by the demo author.