docs: real introductions on index pages and README

docs/manual/index.md

@@ -4,8 +4,10 @@ title: Manual
 
 # Manual
 
-- [Getting started](./getting-started/)
-- [Writing specs](./writing-specs/)
-- [Spec language reference](./spec-language/)
-- [Runs](./runs/)
-- [CLI reference](./cli/)
+- [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.
+- [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.
+- [CLI reference](./cli/): every command and flag.