Add doc for asap templating in public assets

[ryanbethel]

If assets in public folder are retrieved using asap you can include strings like ${STATIC('file.png')} or ${arc.static('file.png')} and those strings will be replaced with the fingerprinted references to that file. I don't think this is in the docs.

This is the only way to reference from one static asset to another when fingerprinting is turned on.

[tbeseda]

I think this fits into the "Using static assets" doc under "App development" in the new outline #411

I'll move this description there, reference this issue, and close here in favor of the outline.