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/syntax-highlighting.md

@@ -0,0 +1,87 @@
+---
+title: Syntax Highlighting
+description: Build-time code highlighting with Tree-sitter
+weight: 3
+---
+
+# Syntax Highlighting
+
+sukr highlights code blocks at build time using Tree-sitter. No client-side JavaScript required.
+
+## Usage
+
+Use fenced code blocks with a language identifier:
+
+````markdown
+```rust
+fn main() {
+    println!("Hello, world!");
+}
+```
+````
+
+## Supported Languages
+
+| Language   | Identifier            |
+| ---------- | --------------------- |
+| Rust       | `rust`, `rs`          |
+| Python     | `python`, `py`        |
+| JavaScript | `javascript`, `js`    |
+| TypeScript | `typescript`, `ts`    |
+| Go         | `go`, `golang`        |
+| Bash       | `bash`, `sh`, `shell` |
+| Nix        | `nix`                 |
+| TOML       | `toml`                |
+| YAML       | `yaml`, `yml`         |
+| JSON       | `json`                |
+| HTML       | `html`                |
+| CSS        | `css`                 |
+| Markdown   | `markdown`, `md`      |
+
+## How It Works
+
+1. During Markdown parsing, code blocks are intercepted
+2. Tree-sitter parses the code and generates a syntax tree
+3. Spans are generated with semantic CSS classes
+4. All work happens at build time
+
+## Styling
+
+Highlighted code uses semantic CSS classes:
+
+```css
+.keyword {
+  color: #ff79c6;
+}
+.string {
+  color: #f1fa8c;
+}
+.function {
+  color: #50fa7b;
+}
+.comment {
+  color: #6272a4;
+}
+.number {
+  color: #bd93f9;
+}
+```
+
+The exact classes depend on the language grammar.
+
+## Nix Language Support
+
+sukr includes full Nix highlighting with injection support. Bash code inside `buildPhase` and similar attributes is highlighted correctly:
+
+```nix
+stdenv.mkDerivation {
+  buildPhase = ''
+    echo "Building..."
+    make -j$NIX_BUILD_CORES
+  '';
+}
+```
+
+## Fallback
+
+Unknown languages fall back to plain `<code>` blocks without highlighting.