Example configuration: memory + rest source

I think a common use case for Orbit will be using the memory source in conjunction with the rest source. A rest source, for newcomers to Orbit, is typically an API that returns data.

The behavior is as follows:
1. Requests are **always** made against the memory source
2. The memory source defers to the rest source when a value is not found
3. Values returned by the rest source are cached in the memory source
4. Subsequent requests for the same resource will use the cached value
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This example should also explore cache invalidation. Two common invalidation strategies I've seen are:
1. automatic invalidation based on time (resources are re-requested after, say, 5 minutes have passed)
2. order invalidation at the time of the request. In pseudocode: `store.find('author', null, {cache: false});`

Start by locating the memory source, rest source, and store.find entry points in the examples repository. Build an example that demonstrates the four stated caching behaviors and explores time-based and request-time invalidation. Done means the example clearly shows fallback, caching, repeated reads, and both invalidation strategies.

Written by the indexing model from the issue text.