Simplify classical shadows tutorial

[natestemen]

The shadows tutorial [live preview, code] covers a ton of material, covering both in-depth theory and the use of the technique. We should trim down the tutorial to go through just the very basics of the classical shadows tutorial, and potentially move some of the theory to the Classical Shadows user guide.¹

Related to #2110

Footnotes

1. This technique currently lacks the "What is the theory behind ...?" page that we have for all the other techniques. ↩