Our UI is heavily disaligned and OUTDATED compared to the Antoras default-ui

[mmattel]

We need to mandatory realign so we can easily integrate changes made by the maintainers from the default-ui into our UI. Our UI was created ~6 years ago and any maintenance (css, js, hbs) is handcrafted since then.

Following stuff needs to be addressed:

• bundle process
• changes in modules used as we use more recent ones
• stuff we created for a reason and is actively used but is not part in the default-ui
• using the supplemental UI capability
• ...