Add Snippets Tutorial

[seanlaw]

The stumpy.snippets feature is now completed in #283 which follows this work:

• The original published paper
• The supporting website with data examples

We have a rough notebook that reproduces the "toy data" example in the original published paper but it lacks the detailed textual context that is consistent with our other existing tutorials.

[NimaSarajpoor]

@seanlaw
I am going to work on this.

[seanlaw]

Sounds good @ninimama !