Stop attempting to comment link to RTD for non-PRs (NOAA-EMC#2209)

Adds a check so comments with a link to documentation are only generated
for PRs.

.github/workflows/docs.yaml

@@ -17,7 +17,7 @@ on:
 jobs:
 
   documentation:
-    
+
     permissions:
       pull-requests: 'write'
 
@@ -56,7 +56,8 @@ jobs:
         path: artifact/doc_warnings.log
         if-no-files-found: ignore
 
-    - name: Comment ReadDocs
+    - name: Comment ReadDocs Link in PR
+      if: github.event_name == "pull_request"
       uses: actions/github-script@v6
       with:
         script: |
@@ -68,5 +69,5 @@ jobs:
            issue_number: context.issue.number,
            owner: context.repo.owner,
            repo: context.repo.repo,
-           body: message 
-          })     
+           body: message
+          })