docs-builder navigation: island nodes, dropdowns, mobile tabs #4338

I made section entries with children render as island tree nodes, added a dropdown section flavor for the top nav in navigation_preview.yml, and added a mobile top-nav tabs menu. I also fixed the synthetics nav tree preservation assertion and reverted a phantom otel-collector entry.

PRs: #3867, #3873, #3874, #3888, #3902.