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/_template/page.html

@@ -32,8 +32,9 @@
     <nav>
       <h3>Manual</h3>
       <ul>
+        <li><a href="__ROOT__manual/introduction/">Introduction</a></li>
+        <li><a href="__ROOT__manual/case-study/">Case study: Folio</a></li>
         <li><a href="__ROOT__manual/getting-started/">Getting started</a></li>
-        <li><a href="__ROOT__manual/writing-specs/">Writing specs</a></li>
         <li><a href="__ROOT__manual/spec-language/">Spec language reference</a></li>
         <li><a href="__ROOT__manual/runs/">Runs</a></li>
         <li><a href="__ROOT__manual/replay/">Replay</a></li>
@@ -52,7 +53,7 @@ $body$
     </article>
     <footer>
       <a href="https://github.com/priyanshujain/sanderling">github.com/priyanshujain/sanderling</a>
-      · <a href="https://github.com/priyanshujain/sanderling/issues/4">v0.1.0 roadmap</a>
+      · <a href="https://github.com/priyanshujain/sanderling/milestone/1">v0.1.0 roadmap</a>
     </footer>
   </main>
 </div>