build(docs-infra): use $schema in ngsw-config.template.json to provide intellisense (#43686)

Add an appropriate `$schema` property in `ngsw-config.template.json` to
allow IDEs to provide intellisense about the various fields of the
config.

PR Close #43686

aio/ngsw-config.template.json

@@ -1,4 +1,5 @@
 {
+  "$schema": "./node_modules/@angular/service-worker/config/schema.json",
   "index": "/index.html",
   "assetGroups": [
     {