chore(docs-gen): create plnkr examples with the correct version

- docs for the snapshot will include the snapshot files from code.angularjs.org
- docs for tagged versions will include the files from the (Google) CDN
- docs for local / untagged versions will try to include the files from the (Google) CDN, which will fail. This gives immediate feedback that something is broken.

Closes #15267
Closes #15358