Themes: offer an opt in prepackaged live preview option for block themes

[annezazu]

Pulling in feedback from the FSE Outreach Program's recent hallway hangout on block themes and the recent revert of a live preview for plugins, I wanted to pass along some feedback that block themers are keen to have a way to offer an easy preview of their block theme. Pulling from the notes:

"@richtabor mentioned trying out a playground blueprint where you can pick patterns, templates, post content, image sources, and have folks dive into the theme to try it out. Perhaps it could reuse the starter content API and extend what’s already there.

We talked about wanting a built in way to demo using WordPress.org that’s so good that theme authors link to that instead of their own demo on their own site."

For reference: ​https://make.wordpress.org/test/2023/10/12/hallway-hangout-lets-chat-about-the-experience-of-building-a-block-theme/

Kicking off this trac issue to explore what this might look like. Similar to the plugin live preview, this should be opt in.

[greenshady]

Some related tickets to theme demos/previews that this proposal could address:

• #4818
• #3731
• #30

[zieladam]

More related resources:

​https://core.trac.wordpress.org/ticket/59416
​https://github.com/WordPress/wordpress-develop/pull/5526
​https://github.com/WordPress/wordpress-playground/pull/700
​https://playground.wordpress.net/wordpress.html