Realign UI and add a managable owncloud overlay

[mmattel]

Continues #864 (Update navigation, related files and some css)

This is a major change for the docs-ui. Though the outcome looks, beside intended changes, the same as it was before, stuff under the hood was changed heavily.

1. Realign to the Antora default UI in the src/ folder for the basic stuff (as of ~ 2025.04.18).
   This includes css, handlebars (hbs in partials), js.
   Note that we already had additional stuff for our UI such as own js or special css files.
   Note that we still are ahead in many package versions that are used by the UI, which is very good.
2. Add an overlay with commented changes that are specific to ownCloud.
   This makes it much easier to update the UI whith changes in the default and to check if and where we need adaptions on our side if used. Or we want to do additions.
3. Removed orphaned files that have not been used at all.
4. Do some fixes that:
   • have been dragged along for a long time and nobody took care such as render issues.
   • have not been implemented but should have been made, mostly css, but also hbs and js.
   • has been implemented incomplete in hbs. I commented it out so we can finalize if necessary.

When doing a file comparison of the src/ folder compared to the Antora default UI, the majority of the files are now contentwise identical again, except where this was not possible due to eslint complains (we have an updated version that is more precise).

With this change, it is now much easier to do fixes, understand changes required for our overlay etc.

I have tested this intensively including full doc builds using local search and different screensizes.

Many thanks to @mzner and @LukasHirt for their support!

[phil-davis]

I don't see any issue, but I'm not reviewing all that JS and CSS in detail!

[mmattel]

I don't see any issue, but I'm not reviewing all that JS and CSS in detail!

Thanks finding the comment typos 😅 👍