[PP-2] Use core's default content system for Umami

Problem/Motivation

Umami includes a bespoke demo_umami_content module which creates the content for the Umami demo. It does this in...some weird way that was necessary at the time Umami was built. A time before we had a default content system in core.

It would be a great simplification of Umami, and an excellent way to really exercise our default content system, by converting Umami's demo content to default content, and hollowing out the demo_umami_content module in favor of just importing all that default content when Umami is installed (this would only be a couple of lines in Umami's install hook).

Remaining tasks

Install Umami, then export all of its content using the default content export system and change Umami to import that content on install.

Since Umami uses Layout Builder, this is blocked on two issues: ✨ [PP-1] Postponed and 📌 [PP-1] Expose Layout Builder data to REST and JSON:API Postponed .