Add example for inlining expanded entities

Add examples for inlining related entities with $expand:

• a non-empty collection in a to-n relation
• an empty collection in a to-n relation
• a single entity in a to-1 relation
• no entity in a to-0-or-1 relation

Could be one example covering all four cases, e.g. two products, with and without a category, and with multiple or no suppliers.