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/configuration.md

@@ -0,0 +1,78 @@
+---
+title: Configuration
+description: Complete reference for site.toml configuration
+weight: 2
+---
+
+# Configuration
+
+sukr is configured via `site.toml`. All settings have sensible defaults.
+
+## Basic Configuration
+
+```toml
+title    = "My Site"
+author   = "Your Name"
+base_url = "https://example.com"
+```
+
+| Field      | Required | Description                      |
+| ---------- | -------- | -------------------------------- |
+| `title`    | Yes      | Site title (used in page titles) |
+| `author`   | Yes      | Author name (used in feeds)      |
+| `base_url` | Yes      | Canonical URL for the site       |
+
+## Path Configuration
+
+All paths are optional. Default values shown:
+
+```toml
+[paths]
+content   = "content"    # Markdown source files
+output    = "public"     # Generated HTML output
+static    = "static"     # Static assets (copied as-is)
+templates = "templates"  # Tera template files
+```
+
+Paths are resolved **relative to the config file location**. This enables monorepo setups:
+
+```bash
+# Build site from subdirectory
+sukr -c sites/blog/site.toml
+# Paths resolve relative to sites/blog/
+```
+
+## CLI Options
+
+```bash
+sukr                           # Use ./site.toml
+sukr -c path/to/site.toml      # Custom config
+sukr --config path/to/site.toml
+sukr -h, --help                # Show help
+```
+
+## Frontmatter
+
+Each Markdown file can have YAML frontmatter:
+
+```yaml
+---
+title: Page Title
+description: Optional description
+date: 2024-01-15 # For blog posts
+weight: 10 # Sort order (lower = first)
+nav_label: Short Name # Override nav display
+section_type: blog # Override section template
+template: custom # Override page template
+---
+```
+
+### Section Types
+
+The `section_type` field determines which template is used for section indexes:
+
+- `blog` → `templates/section/blog.html`
+- `projects` → `templates/section/projects.html`
+- _(any other)_ → `templates/section/default.html`
+
+If not specified, sukr uses the directory name as the section type.