SPT: Serve translated theme homepage templates

[obenland]

Let's explore storing starter page templates and template-first theme's homepage templates in json files to improve parsing times in BlockPreviews and content translations.

See paYJgx-bI-p2.

• Save json in post meta.
• Have a script that looks up post meta and saves into json files.
• Have a lib where these files live + code to save/load/translate.
• Endpoint to pull json files and incorporates translations.
• Find a way to reuse existing template translations.

[obenland]

Since opening this ticket we learned that parsing templates is not our performance bottleneck. Setting state with the parsed blocks, is. See #35713 (comment). So storing them in json files doesn't help much.

For translations, it turns out, we don't need them either. Every theme comes with Headstart annotations that get translated, so we can use those. For template-first themes, those annotations are the same as the demo site content.

Work for using those annotations rather than the demo site content itself is being tracked in D33496-code.

[obenland]

Fixed in D33496-code.