Based on [Zulip discussion](https://code4math.zulipchat.com/#narrow/stream/416467-pi-base/topic/Lean.20integrations/near/406838640). We're [starting to](https://github.com/pi-base/web/pull/86) add links _in_ to Mathlib. We'd like to also supply backlinks from Mathlib properties (&c.) back to the canonical π-Base ID / url.