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

docs/02-concepts/01-arrays-and-strings.mdx

@@ -63,3 +63,9 @@ Random access in this context means that you can access an element at any index
 </Columns>
 
 
+
+## Study deck
+
+Drill the essentials — click the deck, then use the arrow keys.
+
+<StudyDeck deck="arrays-and-strings" />