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/_index.md

@@ -0,0 +1,35 @@
+---
+title: sukr
+description: Minimal static site compiler — suckless, Rust, zero JS
+---
+
+# Welcome to sukr
+
+**sukr** transforms Markdown into high-performance static HTML. No bloated runtimes, no client-side JavaScript, just clean output.
+
+## Why sukr?
+
+- **Fast builds** — Single Rust binary, parallel processing
+- **Zero JS** — Syntax highlighting at build time via Tree-sitter
+- **Flexible templates** — Runtime Tera templates, no recompilation
+- **Monorepo-ready** — Multiple sites via `-c` config flag
+
+## Quick Start
+
+```bash
+# Install
+cargo install sukr
+
+# Create site structure
+mkdir -p content templates static
+echo 'title = "My Site"' > site.toml
+echo 'author = "Me"' >> site.toml
+echo 'base_url = "https://example.com"' >> site.toml
+
+# Build
+sukr
+```
+
+## Documentation
+
+Browse the sidebar for detailed documentation on all features.