Add support for content filtering

Issue

Add content parameter to filter methods and add a contents property for retrieving unique content values in a collection.

[x] Read through all changes. No redundant print() statements, commented-out code, or other remnants from the development. 👀
[x] Make sure tests pass after every commit. ✅
[x] New/refactored code is following same conventions as the rest of the code base. 🧬
[x] New/refactored code is tested. ⚙
[x] Documentation has been updated 🧾