[meta] Automatic Site Template Generation

Problem/Motivation

The more I think about Site Templates, and the more I talk to people about it, the more convinced I become of this: Site Templates should be something designers can build. Ideally, it would work like this: you go into Drupal and Experience Builder, design a beautiful site or theme, and simply click “Export as Site Template.” That is how we are going to get the best templates. If we require template builders to write YAML files, I worry we will attract the wrong audience and get subpar templates. Thoughts? -- Dries in Drupal Slack

Proposed resolution

Devise a plan to create a Site template in the UI. These Site templates are to include, but not be limited to:

A composer template generator
Experience Builder (XB) component exporter/UI
Drupal Theme generator/UI
Recipe exporter/UI
Default content exporter UI (Exporter exists in contrib in the Default Content module)

Remaining tasks

All of the above.

User interface changes

TBD

Introduced terminology

Site template

API changes

TBD

Data model changes

TBD

Release notes snippet