fix(menu_item): use item title for id if there's no path

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

Resolves #2006 (closed)

Some nav categories on the doc site don't have standalone pages (so, no path value in nav.json). As a result, the itemId is item-undefined for a number of nested menu ids and their corresponding aria-controls attributes.

This change will use the category's title to create a unique itemId for the category's menu_item if there isn't a value for path coming from nav.json for that given category.

I also updated the sidebar snapshot test to reflect this change.