Add section about the design of CPython's garbage collector (#562)

Co-Authored-By: Tim Peters <[email protected]>
Co-Authored-By: Terry Jan Reedy <[email protected]>
Co-authored-by: Brett Cannon <[email protected]>

Author: 4 people

appendix.rst

@@ -51,6 +51,7 @@ Language development in depth
 * :doc:`exploring`
 * :doc:`grammar`
 * :doc:`compiler`
+* :doc:`garbage_collector`
 * :doc:`stdlibchanges`
 * :doc:`langchanges`
 * :doc:`porting`
@@ -64,4 +65,4 @@ Testing and continuous integration
 * :doc:`buildbots`
 * :doc:`buildworker`
 * :doc:`coverity`
-  
+  

garbage_collector.rst

@@ -260,6 +260,7 @@ Additional Resources
     * :doc:`exploring`
     * :doc:`grammar`
     * :doc:`compiler`
+    * :doc:`garbage_collector`
 * Tool support
     * :doc:`gdb`
     * :doc:`clang`
@@ -317,6 +318,7 @@ Full Table of Contents
    exploring
    grammar
    compiler
+   garbage_collector
    extensions
    coverity
    clang