Add Triggers v0.21.0

Add the docs for Triggers v0.21.0

sync/config/triggers.yaml

@@ -26,6 +26,17 @@ archive: https://github.com/tektoncd/triggers/tags
 #       header: <dict>         # optional, no header added if not set
 #         See https://www.docsy.dev/docs/adding-content/navigation/#section-menu
 tags:
+- name: release-v0.21.x
+  displayName: v0.21.0
+  # Dict of folders to sync
+  folders:
+    docs:
+      index: README.md
+      include: ['*.md']
+    docs/getting-started:
+      target: getting-started
+      include:
+      - create-ingress.yaml
 - name: v0.20.0
   displayName: v0.20.0
   # Dict of folders to sync