Themed starts: replace the stale theme branches with reusable theme packages

[Decipher]

What is happening

This starterkit has advertised two themed variants as branches:

• theme/bootstrap-vue
• theme/tailwindcss

Both are frozen on a November 2022 version of the repository. They are
Drupal 9 era and contain none of the Drupal 11 upgrade, the Docker-free
local backend, the dev container, or the one-command setup.

Anyone starting from those branches today gets the old starterkit, so
the README no longer links them.

Why they went stale

Each branch was regenerated by a workflow that ran on every push to
develop, installed the theme, and force-pushed the result. The
mechanism worked, but it only fires when develop moves, and develop
had not moved since 2022. The workflows have now been removed: they
failed on a deprecated action, and their theme install step edited a
line of nuxt.config.js that no longer exists, so repairing them would
have produced branches labelled as themed that carried no theme.

What replaces them

Each theme is a very small delta - a dependency and a line or two of
configuration. Carrying that as a whole branch per theme, per
starterkit, does not scale and rots silently, as this did.

The direction is to publish themes as reusable packages that any Druxt
site can install, rather than as branches you start from. That also
makes them usable by existing sites, which branches never were.

Meanwhile

The branches are left in place for anyone already using them. Adding a
UI framework to a fresh install is the normal Nuxt process: install the
module and add it to modules in nuxt/nuxt.config.js.

Progress will be tracked here.