Fork Antora Default UI and decide on/implement desired changes

[d-spencer-strickland]

• Get rid of sample content

• Other changes TBD

[d-spencer-strickland]

We'll need to continue to maintain our fork with upstream changes ongoing, but activity is light so shouldn't pose any issues.

[alandefreitas]

Why fork the default antora UI from scratch again instead of starting with what we already have here in site-docs?