Add preview.html file to showcase a demo version of the theme

[bradhogan]

What problem does this address?

Currently, the only way to have something that resembles a decent preview of a theme is to add a front-page.html or home.html template to your block theme and stack it with patterns. This proposes a rather confusing experience for someone when they go to edit their homepage content because they have to set the front page display as a page, but then that page is just a placeholder. They then have to go into the site editor and edit the front page template.

What is your proposed solution?

I suggest we allow block theme authors to create a file inside the root theme directory called preview.html where we can add patterns, etc. to create a look that we think represents our design well. This file would have no editing features or front-end display other than to simply be what is displayed if a person clicks "preview" to preview the theme. And if preview.html does not exist, the behavior just simply defaults to the original front-page.html, home.html or index.html.