build(docs-infra): add support for examples of type elements (#24840)

Examples using `@angular/elements` need to transpile to es2015 for
Custom Elements to work (on browsers that natively support them).

Alternatively, a polyfill would need to be loaded. For now, changing the
transpilation target to es2015 is the simplest solution.

PR Close #24840

aio/content/examples/elements/example-config.json

@@ -0,0 +1,3 @@
+{
+  "projectType": "elements"
+}