wip: Added stub for what will be composite index docs

Author: colinbarry

pages/fundamentals/indexes.mdx

@@ -134,6 +134,10 @@ expression, the expression can be so complicated that the index doesn't get used
 If there is any suspicion that an index isn't used, we recommend writing
 labels and properties inside the `MATCH` pattern.
 
+### Label-properties (composite) index
+
+An index on a label and one or more properties is called a composite index. 
+
 ### Edge-type index
 
 To optimize queries that fetch only the edges by specific edge-types, you need