Add ability to collapse sections by default separately

[frangio]

It's now possible to choose specific sections in the sidebar navigation that should be collapsed by default, as opposed to all sections.

In antora.yml for the component, set the attribute page-sidebar-collapse-default to a string of comma-separated navigation sections:

asciidoc: 
  attributes:
    page-sidebar-collapse-default: 'Tutorials,Other'

If nav.adoc looks like below, only the Tutorials section will start collapsed.

* xref:index.adoc[Overview]

.Reference
* xref:javascript.adoc[JavaScript]
* xref:perl.adoc[Perl]

.Tutorials
* xref:a-tutorial.adoc[A Tutorial]
* xref:another-tutorial.adoc[Another Tutorial]

CC @cairoeth