Validation tests for embedded examples in the new book

The Bevy Book chapters have lots of embedded examples in doctest format, but many of them have errors - either syntax errors or calling APIs that don't exist. We should have some sort of test that we can run locally to ensure the integrity of the examples.

Start by surveying the Bevy Book chapters and their embedded doctest examples to determine how the examples are currently represented and run locally. Done means a repeatable local test detects examples with syntax errors or calls to APIs that do not exist.

Written by the indexing model from the issue text.