Docs examples 11/14: Rework navigation component examples

Part 11 of 14 in the stack that splits up the approved draft !6295 (closed) into reviewable pieces. The content is exactly what was reviewed there; I've only sliced it.

Related to #3575.

Pages: breadcrumb, tabs, pagination, path, nav item, link, collapse, accordion. Realistic content and contextual placement. Tabs also loses two duplicate Code reference examples, and accordion gains an auto-collapse example under GlAccordion.

Safe to publish without the MRs after it.