portico: Link back to /for/X pages from Lean case study.

Author: alya

templates/zerver/for/lean-case-study.md

@@ -129,3 +129,9 @@ jump in to help newer community members as time allows.
 > — [Robert Y. Lewis](https://robertylewis.com/), Brown University lecturer and
 > [mathlib](https://github.com/leanprover-community/mathlib)
 > [maintainer](https://github.com/leanprover-community/mathlib#maintainers)
+
+---
+
+Check out our guides on using Zulip for [research](/for/research)
+and [open source](/for/open-source), and learn how Zulip
+[helps communities grow](/for/communities)!