Tutorial Gallery - put headers in own cells so they render better in readthedocs

[Steph-Chong]

Have submitted PR #572.

This PR puts the headers in the Tutorial Gallery into their own cells. This way, they render better in readthedocs. In particular the second level headers (e.g. Continuous, Probability etc.) will show up in the contents in the right hand side bar, making the page easier to navigate (especially as the number of tutorials continues to grow).

(At the moment, no header levels show up in the right hand side bar - the "Contents" is empty - on readthedocs).