Rewrite docs and update mdbook theme for 0.5.2

Rewrite all doc pages to be accurate and concise:
- introduction.md: explains the actual purpose (type safety overlay,
  not a new interface), real CLI commands
- type-mapper.md: focused reference with mapping tables
- examples.md: fixed generated code to match actual output (removed
  phantom .instance path, wrong nullOr wrapping, nonexistent
  config/... wrapper)
- design-overview.md: condensed from 540 to 85 lines, pipeline
  diagram, module table, key design decisions

Remove pages for unbuilt features:
- option-builder.md (internal API docs with wrong test counts)
- documentation-generation.md (680-line plan for unimplemented feature)

Update index.hbs to stock mdbook 0.5.2 template (old one used
removed {{#previous}}/{{#next}} block helpers) with catppuccin
theme buttons.

Author: typedrat

.gitignore

@@ -18,3 +18,6 @@ result-*
 
 # treefmt-nix
 .treefmt.toml
+
+# mdbook
+docs/book/

docs/src/SUMMARY.md

@@ -2,13 +2,6 @@
 
 [Introduction](./introduction.md)
 
-# Design
-
-- [Design Overview](./design-overview.md)
+- [Type Mapping](./type-mapper.md)
 - [Transformation Examples](./examples.md)
-- [Documentation Generation](./documentation-generation.md)
-
-# Implementation
-
-- [Type Mapper](./type-mapper.md)
-- [Option Builder](./option-builder.md)
+- [Architecture](./design-overview.md)