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/models/index.md

@@ -0,0 +1,6 @@
+# Models
+
+::: docforge.models
+- [Module](module.md)
+- [Object](object.md)
+- [Project](project.md)

docs/lib/docforge/models/module.md

@@ -0,0 +1,3 @@
+# Module
+
+::: docforge.models.module

docs/lib/docforge/models/object.md

@@ -0,0 +1,3 @@
+# Object
+
+::: docforge.models.object

docs/lib/docforge/models/project.md

@@ -0,0 +1,3 @@
+# Project
+
+::: docforge.models.project