feat: add wiki build kind with file-structure-derived navigation

- add build_wiki_nav deriving MkDocs nav from docs/wiki file structure
  (index.md -> Home, numeric prefixes stripped and title-cased, nested
  dirs become groups, natural ordering)
- add --wiki / --wiki-dir to build; wiki-only builds need no --module
- merge wiki nav before generated lib/api nav; wiki Home replaces the
  nav spec Home entry
- add mkdocs.wiki.yml template fragment and nav/cli tests
- dogfood doc-forge's own docs/wiki and regenerate site output

docs/wiki/01_overview.md

@@ -0,0 +1,25 @@
+# Overview
+
+`docforge` turns GSDFC-compliant Python docstrings into maintainable reference
+documentation. It never edits source docstrings; it reads them, renders them,
+and assembles a single MkDocs site from all available material.
+
+## What it generates
+
+| Kind      | Source                     | Output                          |
+|-----------|----------------------------|---------------------------------|
+| `lib`     | GSDFC docstrings           | `docs/lib/**` rendered markdown |
+| `api`     | OpenAPI JSON spec          | `docs/api/**` rendered markdown |
+| `wiki`    | Hand-written markdown      | `docs/wiki/**` (unchanged)      |
+| `mcp`     | Griffe + renderers         | `docs/mcp/**` structured files  |
+
+## Combined build
+
+One `mkdocs.yml` and one MkDocs build serve all kinds:
+
+1. Wiki navigation is derived from the `docs/wiki/` file structure.
+2. Generated library/API navigation is appended after it.
+3. The wiki `index.md` becomes the site `Home`.
+
+Hand-written wiki content is never overwritten or regenerated — only its
+navigation is derived automatically.