docs(docs): add study decks, new reference pages, and fix header image path

docs/02-concepts/02-hashing.mdx

@@ -28,3 +28,9 @@ Appending to the end of a list is amortized O(1) time complexity. This means tha
 Random access in this context means that you can access an element at any index in constant time.
 :::
 
+
+## Study deck
+
+Drill the essentials — click the deck, then use the arrow keys.
+
+<StudyDeck deck="hashing" />