Add notify-element-web job (#7062)

This triggers a repository_dispatch event in element-web which allows that repo to run new CI actions for EW with the new SDK code

.github/workflows/notify-element-web.yml

@@ -0,0 +1,15 @@
+name: Notify element-web
+on:
+    push:
+        branches: [develop]
+jobs:
+    notify-element-web:
+        runs-on: ubuntu-latest
+        environment: develop
+        steps:
+            -   name: Notify element-web repo that a new SDK build is on develop
+                uses: peter-evans/repository-dispatch@v1
+                with:
+                    token: ${{ secrets.ELEMENT_WEB_NOTIFY_TOKEN }}
+                    repository: vector-im/element-web
+                    event-type: element-web-notify