docs: reorganize into project/ + reference/, expand Storybook Foundations

Move working docs (backlog, prd, roadmap) under docs/project/ and durable
docs (architecture ADRs, guides, audits) under docs/reference/; add a
docs/README.md index. Update every path reference in code comments, CLAUDE.md,
READMEs, and the new-ssp skill. Expand the Storybook Foundations curriculum
(Overview, BDD, i18n; rename Layers→Domain-Driven Design) and reorder the sidebar.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/BigRegister.Api/Domain/Letters/LetterHtml.cs

@@ -150,7 +150,7 @@ public static class LetterHtml
   private static string EncLines(string s) => Enc(s).Replace("\n", "<br>");
 
   // Walks up from the running assembly's own directory (NOT the process cwd, which
-  // varies by how `dotnet run`/docker/tests invoke it — see docs/backlog/WP-25) until
+  // varies by how `dotnet run`/docker/tests invoke it — see docs/project/backlog/WP-25) until
   // it finds `public/letter.css`. docker-compose.yml bind-mounts `./public` under the
   // api container's `/src` for exactly this walk to resolve there too.
   private static string FindLetterCss()