docs: point manual navigation at the case study

docs/manual/index.md

@@ -5,8 +5,8 @@ title: Manual
 # Manual
 
 - [Introduction](./introduction/): what property-based testing is and how sanderling works.
-- [Getting started](./getting-started/): install and run a first test against the example app.
-- [Writing specs](./writing-specs/): build a real spec from scratch, one concept at a time.
+- [Case study: Folio](./case-study/): sanderling finding a real bug in a mobile app, and how the spec is written.
+- [Getting started](./getting-started/): install the CLI and run it against Folio.
 - [Spec language reference](./spec-language/): every selector, operator, action, and sampler.
 - [Runs](./runs/): what happens during a run and why runs are long.
 - [Replay](./replay/): the trace browser.