build: sign commits in REPL docs workflow

.github/workflows/update_repl_docs.yml

@@ -105,6 +105,16 @@ jobs:
         run: |
           rm -rf .git/hooks
 
+      # Import GPG key to sign commits:
+      - name: 'Import GPG key to sign commits'
+        # Pin action to full length commit SHA
+        uses: crazy-max/ghaction-import-gpg@01dd5d3ca463c7f10f7f4f7b4f177225ac661ee4 # v6.1.0
+        with:
+          gpg_private_key: ${{ secrets.STDLIB_BOT_GPG_PRIVATE_KEY }}
+          passphrase: ${{ secrets.STDLIB_BOT_GPG_PASSPHRASE }}
+          git_user_signingkey: true
+          git_commit_gpgsign: true
+
       # Create a pull request:
       - name: 'Create pull request'
         id: cpr
@@ -118,7 +128,8 @@ jobs:
             -   updates the REPL namespace documentation
 
           commit-message: 'docs: update REPL namespace documentation'
-          committer: 'stdlib-bot <[email protected]>'
+          committer: 'stdlib-bot <[email protected]>'
+          signoff: true
           token: ${{ secrets.PULL_REQUEST_TOKEN }}
           labels: |
             documentation