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/nav/index.md

@@ -0,0 +1,7 @@
+# Nav
+
+::: docforge.nav
+- [Mkdocs](mkdocs.md)
+- [Resolver](resolver.md)
+- [Spec](spec.md)
+- [Wiki](wiki.md)