Improve notebooks in examples

[elijahbenizzy]

All notebooks should:

1. Explain what you'll learn
2. Display/link to the code they pull in
3. Explain what they're doing
4. Explain why they use specific APIs
5. Link to docs

Continuation of #114

[andreahlert]

Labels migrated to new taxonomy (#692).

[andreahlert]

Closing as stale. This is too broad to be actionable. Specific notebook improvements can be tracked as individual issues.