Allow large tables to overlay the right-side TOC

Allow a large table to overlay the right-hand side ToC.

See https://asciidoctor.zulipchat.com/#narrow/stream/279642-users/topic/horizontal.20scroll.20for.20large.20tables/near/321055258 and https://antora.zulipchat.com/#narrow/stream/282400-users/topic/.E2.9C.94.20Body.20content.20overlapping.20right-hand.20side.20TOC