docs: point manual navigation at the case study

docs/manual/introduction.md

@@ -82,9 +82,9 @@ Kotlin Multiplatform apps need nothing special: the Android build is tested thro
 
 ## Reading this manual
 
-- [Getting started](../getting-started/) installs the CLI and runs a first test against folio, the example app.
-- [Writing specs](../writing-specs/) builds the folio spec from scratch and teaches every concept along the way.
-- [Spec language reference](../spec-language/) lists every selector, operator, and action.
+- [Case study: Folio](../case-study/) follows sanderling finding a real bug in the example app, and shows how its spec is written.
+- [Getting started](../getting-started/) installs the CLI and runs it against Folio.
+- [Spec language reference](../spec-language/) lists every selector, operator, action, and sampler.
 - [Runs](../runs/) explains what happens during a run and why runs are long.
 - [Replay](../replay/) covers the trace browser.
 - [CLI reference](../cli/) lists every command and flag.