Consider dropping the use of details/summary in the navbar

The following discussion from !2371 (merged) should be addressed:

• @jeldergl started a discussion: (+3 comments)

  The use of details and summary here feels out of place semantically. I need to test this in a screen reader, but I'm wondering what the reason is for using it?

The navbar currently leverages <details> and <summary> elements to render collapsible menu items. This might not be a valid use-case for those elements, let's consider dropping them in favor of simple v-ifs in the Vue components.

Implementation plan

• Drop the use of summary elements in favor uls and v-ifs.
• Do not automatically select the first child item when clicking on a parent.
• Keep the behavior where only one menu section can be expanded at a time.
• Improve navbar's accessibility by leveraing role and aria- attributes.