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/site.toml

@@ -0,0 +1,9 @@
+author   = "Sukr Contributors"
+base_url = "https://sukr.io"
+title    = "Sukr Documentation"
+
+[paths]
+content   = "content"
+output    = "public"
+static    = "static"
+templates = "templates"