Skip tempest jobs for doc/* only changes

Neutron repo contains doc/ directory to store install and other guides.
We should not execute tempest jobs if a patch touches those files only.

Change-Id: If5003bb573e959a2324eab62cc08822c6e28e5f4

.zuul.yaml

@@ -12,6 +12,7 @@
     irrelevant-files:
       - ^(test-|)requirements.txt$
       - ^releasenotes/.*$
+      - ^doc/.*$
       - ^setup.cfg$
 
 - job:
@@ -28,6 +29,7 @@
     irrelevant-files:
       - ^(test-|)requirements.txt$
       - ^releasenotes/.*$
+      - ^doc/.*$
       - ^setup.cfg$
     voting: false
 
@@ -46,6 +48,7 @@
     irrelevant-files:
       - ^(test-|)requirements.txt$
       - ^releasenotes/.*$
+      - ^doc/.*$
       - ^setup.cfg$
     voting: false