Update hero and staggered animation pages with code excerpts and tests

[parlough]

The Hero animations and Staggered animations pages were out of date as they are not currently using code excerpts and they don't have tests. They have analyzed code snippets in the examples/_animations directories but they are not tied to them.

The pages should be:

• [ ] Determined if worthwhile to keep around
• [ ] Text and code snippets reviewed for accuracy
• [ ] Tied to existing or new code excerpts
• [ ] At least basic tests added

[atsansone]

Related to https://github.com/flutter/website/issues/5792.