Document how to specify a supplemental UI using the ui.supplemental_ui key in the playbook
Antora supports overlaying additional files onto the loaded UI bundle, which we call the supplemental UI. These files are configured using the ui.suppemental_files
key in the playbook. For awhile, this feature was considered experimental. There's been enough real world usage of it that we know now that it's a feature worth keeping. We'd like to mark this feature as stable in Antora 3.
The value of the supplemental_files
key can either be a directory on the filesystem where the supplemental files are stored, or an array of virtual files that are defined individually. Each virtual file can source its contents from a file on the filesystem. These two value types are mutually exclusive. The documentation should illustrate the difference.
Since defining static files using the UI is closely related to this feature, the page should also document how to use the supplemental UI to create static (non-UI) files by declaring them in ui.yml.