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/lib/docforge/renderers/base.md

@@ -0,0 +1,3 @@
+# Base
+
+::: docforge.renderers.base

docs/lib/docforge/renderers/index.md

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+# Renderers
+
+::: docforge.renderers
+- [Base](base.md)
+- [Mcp Renderer](mcp_renderer.md)
+- [Mkdocs Renderer](mkdocs_renderer.md)

docs/lib/docforge/renderers/mcp_renderer.md

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+# Mcp Renderer
+
+::: docforge.renderers.mcp_renderer

docs/lib/docforge/renderers/mkdocs_renderer.md

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+# Mkdocs Renderer
+
+::: docforge.renderers.mkdocs_renderer