Add Ansible 13 porting guide to index. (#3273) (#3274)

(cherry picked from commit 1913656)

Co-authored-by: Felix Fontein <[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_13
    porting_guide_12
    porting_guide_11
    porting_guide_10