Agreed. There are some public building blocks available (e.g. Kythe or meta's Glean) but having something generic that produces the kind of experience you can get on cs.chromium.org seems impossible. You need such bespoke build integration across an entire organization to get there.
Basic text search, as opposed to navigation, is all you'll get from anything out of the box.
Basic text search, as opposed to navigation, is all you'll get from anything out of the box.