Merge pull request #3832 from shaedrich/topic/cst

Add topic for "cst"

Author: zkoppert

topics/cst/index.md

@@ -0,0 +1,9 @@
+---
+display_name: Concrete syntax tree
+short_description: A concrete syntax tree is a low level representation of the parsed source in the structure defined by a grammar description.
+topic: cst
+aliases: parse-tree, parsing-tree, derivation-tree
+wikipedia_url: https://en.wikipedia.org/wiki/Parse_tree
+related: ast
+---
+A concrete syntax tree is an ordered, rooted tree that represents the syntactic structure of a string according to some context-free grammar.