boost look does not support collapsible ToC

[Becheler]

Hi,

In the Boost.Graph preview, the table of content is not collapsed properly: https://www.boost.org/doc/libs/develop/doc/antora/graph/index.html

This is quite a bother because for large/complex libs we expect the ToC to have:

1. More than two nested levels
2. Too many categories to be flat

This makes visitors have to scroll over the toc to identify relevant sections, which is painful.

For contrast, I tweaked locally to be something like this: https://491.graph.prtest3.cppalliance.org/graph/index.html

The relevant code to achieve this is located in https://github.com/boostorg/graph/tree/develop/doc