Merge pull request ansible#2628 from samccann/port-guide-index (ansible#2633)

add 12 porting guide to the index

(cherry picked from commit 220cae6)

Co-authored-by: Alicia Cozine <[email protected]>

Author: patchback[bot]

docs/docsite/rst/porting_guides/porting_guides.rst

@@ -10,6 +10,7 @@ This section lists porting guides that can help you in updating playbooks, plugi
    :maxdepth: 1
    :glob:
 
+   porting_guide_12
    porting_guide_11
    porting_guide_10
    porting_guide_9