Sync list of tutorial cards and left-hand nav section (#922)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/922

Make sure that the list of tutorials in the left-hand nav section agrees with the set of tutorial cards.

Rearrange the tutorials to do more general ones first, then mobile apps (which will probably have a larger audience), then an alphabetical list of backend examples.

Reviewed By: mergennachin, kimishpatel

Differential Revision: D50302244

fbshipit-source-id: a1431264cfdbb983cf6b9ad116eac83fb70c120e