[sidebar] Offer the possibility to expand a collapsible section if we are navigating to a children page

[camilleterol]

When a section of the sidebar is set with collapsed to true, it is sometimes hard to understand that the page you are browsing is in this section when you have followed a link.

Would it be possible to have an option to expand a section if the active page is a children of the section ?
Maybe adding a new setting to collapsed for example collapsed: unless-active or something of the sort?

[brc-dd]

It's the default behavior if you have collapsed: true. There's a bug with it not working for nested children. Should be fixed by #1361.