Legacy docs should display proper styling without boostbookV2

[cdw9]

I separated this out from #1459

Make sure pages like https://www.stage.boost.cppalliance.org/doc/libs/release/libs/utility/doc/html/utility/utilities/call_traits.html will still look good after boostbookv2.css is removed

[daveoconnor]

This may be related

https://github.com/boostorg/website-v2/pull/1460