Add sig-doc-automation Slack Channel

Author: allenfeng8

communication/slack-config/sig-docs/docs-channels.yaml

@@ -21,6 +21,7 @@ channels:
   - name: kubernetes-docs-pl
   - name: kubernetes-docs-uk
   - name: sig-docs
+  - name: sig-docs-automation
   - name: sig-docs-blog
   - name: sig-docs-dockershim
     archived: true