Version and publish UI bundle to a package repository

The default UI started out as a template project for creating a custom UI. However, when we started publishing the produced UI bundle as a CI artifact, users (including the Antora project itself) started hot linking to it as a release. This situation is problematic for several reasons:

• The URL of the published artifact is awkward
• During a build, the URL becomes unavailable
• The bundle is not versioned, so every change invokes a rolling update
• Lack of versioning is locking us into supporting IE and ES5 (since those runtimes were supported when the bundle was first made available)
• We can't rename the default branch from master to main because it changes the URL of the published artifact

There's also the problem that the name of the project is misleading. This project is really meant to be a reference for how to make a UI rather than a default. So we might consider renaming it.

One way to version and publish the UI is to publish it as an npm package. That would allow the UI to be installed alongside Antora. Since Antora now supports loading the UI bundle from a directory, it can point to the installed path of the package. However, a lot of users will probably still prefer to linking to the UI from a public URL, so we will need to think about where to publish it. We could use the package and/or releases facility in this repository as that can be assumed to be more stable, certainly more so than a CI artifact.