fix: ui demo, and make it an opt-in feature

[wesleyboar]

Overview / Changes

UI Demo:

• becomes dev feature in Core CMS
• becomes opt-in feature for client CMS
• renamed to ui-demo (not css-demo)
• documented as all the above

Important Closed, because it adds too much time to client build.

Related

• fixes #692
• required by https://github.com/TACC/tup-ui/pull/236

Testing

1. Build fresh local Core CMS.
2. Build demo:

   docker exec -it core_cms /bin/bash
   # That opens a command prompt within the container.
       npm run build:ui-demo

3. Give server time to auto-restart.
4. Open demo: http://localhost:8000/static/ui/index.html
5. Visit CMS pattern, like Django Forms: http://localhost:8000/static/ui/components/detail/django-cms-forms.html
6. Verify form is styled.

UI

Local Core (TACC/Core-CMS) - Manual Demo Build	Local Client (TACC/tup-ui) - Opt-In Demo Build

Remote Client (dev.tup.tacc…) - Opt-In Demo Build	Remote Client (dev.cep.tacc…) - No Demo Build

[wesleyboar]

This adds too much time to the client build.

I think there is a better way to not build for every CMS, but also cache the demo (perhaps isolate its build).