Add "When Using Webpack..." to "Getting Started"

[webdiscus]

Summary

Proposed change:

Added information to the Getting Started site about using Webpack to compile nunjucks templates.

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• [x] Proposed change helps towards purpose of this project.
• [x] Documentation is added / updated to describe proposed change.
• [ ] Tests are added / updated to cover proposed change.
• [ ] Changelog has an entry for proposed change (if user-facing fix or feature).