Document licenses for third party open source projects that we depend on

Created by: nicksnyder

We need something like this: https://neo4j.com/terms/thirdpartylicenses/

The page should be generated automatically. Tagging both @lguychard and @tsenart because the implementation is probably different for TypeScript and Go dependencies.

I would like to prioritize this for 3.15 release on April 20 but if you want to do it earlier, I wouldn't complain :)

slack thread: https://sourcegraph.slack.com/archives/C0B2RU51Q/p1582918787128100