Document copying of assets

[rschristian]

Discussed in https://github.com/preactjs/preact-cli/discussions/1610

It's pretty common in some other build tools to have a public/ directory from where assets are copied.

We could do with replicating this behavior (even if we don't provide a public/ dir in any of the templates) or at least providing some better documentation for it. I believe the only source of information at the moment is in the source.