Add a note about python in gdb

_gtfobins/gdb.md

@@ -1,4 +1,10 @@
 ---
+description: |
+  GDB may come with embedded Python support, in that case arbitrary code can be
+  executed with the `python` command in the context of the GDB process.
+
+  See the entries of Python version [2](/gtfobins/python2/) and version
+  [3](/gtfobins/python3/).
 functions:
   execute-interactive:
     - code: gdb -nx -ex '!sh' -ex quit