Ruby: AST ref docs - add too toctree

Author: alexrford

docs/codeql/codeql-language-guides/codeql-for-ruby.rst

@@ -10,6 +10,7 @@ Experiment and learn how to write effective and efficient queries for CodeQL dat
 
    basic-query-for-ruby-code
    codeql-library-for-ruby
+   abstract-syntax-tree-classes-for-working-with-ruby-programs
 
 -  :doc:`Basic query for Ruby code <basic-query-for-ruby-code>`: Learn to write and run a simple CodeQL query using LGTM.