docs update with case study (#63)

* docs(manual): add introduction page

* docs(manual): rewrite getting started as guided first run

* docs(manual): rewrite writing specs as a folio tutorial

* docs(manual): document missing spec API in reference

* docs(manual): plain-language rewrite of runs page

* docs: real introductions on index pages and README

* fix(docs): sibling links from directory-style pages need ../

* fix(docs): correct sampling and restart-cost claims to match implementation

* docs: nav lists Introduction and Case study; roadmap points to milestone

* docs(manual): make getting started target the reader's own app, not Folio

* docs(manual): add Folio case study page

* docs: point manual navigation at the case study

* docs(readme): lead with the case study, fix roadmap link

* docs: roadmap links to milestone, sync clear-data default and cross-links

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.
+- [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.
+- [CLI reference](./cli/): every command and flag.