Add 'contribute-docs' to navigation menu

🐔🥚: elastic/docs-content#3230

Author: leemthompo

config/navigation.yml

@@ -22,6 +22,7 @@ toc:
   - toc: deploy-manage
   - toc: cloud-account
   - toc: troubleshoot
+  - toc: contribute-docs
 
   ##########
   # EXTEND #