feat(docs): create sukr documentation site

Self-documenting docs site built with sukr itself (dogfooding):
- docs/site.toml with sukr.io base URL
- docs-specific templates with sidebar navigation
- Dark theme CSS, responsive layout
- Documentation: getting-started, configuration, features

Also: improved error chaining for better template debugging

docs/content/features/_index.md

@@ -0,0 +1,8 @@
+---
+title: Features
+description: Explore sukr's capabilities
+section_type: features
+weight: 3
+---
+
+sukr provides a focused set of features for building fast, minimal static sites.