Docs Request: "Features" page about the `dy.Collection` type

Now that there's a "Features" section in the docs, it might be helpful to cover the dy.Collection class in there, with a focus more on "what are some common workflows" (as opposed to the existing example in "Real-world Example", and the noisy API documentation generated for that class).

Some question to think about that might be relevant:

1. How do you define foreign key relationships in a dy.Collection data model? Is it done automatically by column name? Is there a way to do it manually?
2. How do you specify the cardinality of join validations (e.g., 1:1, 1:m, etc.).
3. How do you specify whether nulls are treated as a "value" in the join, vs. the absense of data? SQL always takes the former approach, but Polars is nice in that it lets you select whether null is a value itself or a marker for missing/unknown data (e.g., by its nulls_equal= argument).

Start with the existing Features section, the Real-world Example, and the generated API documentation for dy.Collection. Review the Collection behavior behind foreign-key relationships, join cardinality, and null handling, then document common workflows and clearly answer those questions in the new page.