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/templates.md

@@ -0,0 +1,91 @@
+---
+title: Tera Templates
+description: Customizable templates without recompilation
+weight: 1
+---
+
+# Tera Templates
+
+sukr uses [Tera](https://tera.netlify.app/), a Jinja2-like templating engine. Templates are loaded at runtime, so you can modify them without recompiling sukr.
+
+## Template Directory Structure
+
+```
+templates/
+├── base.html               # Shared layout (required)
+├── page.html               # Standalone pages
+├── homepage.html           # Site homepage
+├── section/
+│   ├── default.html        # Fallback section index
+│   ├── blog.html           # Blog section index
+│   └── projects.html       # Projects section index
+└── content/
+    ├── default.html        # Fallback content page
+    └── post.html           # Blog post
+```
+
+## Template Inheritance
+
+All templates extend `base.html`:
+
+```html
+{% extends "base.html" %} {% block content %}
+<article>
+  <h1>{{ page.title }}</h1>
+  {{ content | safe }}
+</article>
+{% endblock content %}
+```
+
+## Available Context Variables
+
+### All Templates
+
+| Variable        | Description                     |
+| --------------- | ------------------------------- |
+| `config.title`  | Site title                      |
+| `config.author` | Site author                     |
+| `nav`           | Navigation items                |
+| `page_path`     | Current page path               |
+| `prefix`        | Relative path prefix for assets |
+| `base_url`      | Canonical base URL              |
+| `title`         | Current page title              |
+
+### Page Templates
+
+| Variable           | Description           |
+| ------------------ | --------------------- |
+| `page.title`       | Page title            |
+| `page.description` | Page description      |
+| `content`          | Rendered HTML content |
+
+### Section Templates
+
+| Variable              | Description                       |
+| --------------------- | --------------------------------- |
+| `section.title`       | Section title                     |
+| `section.description` | Section description               |
+| `items`               | Array of content items in section |
+
+### Content Item Fields (in `items`)
+
+| Variable           | Description         |
+| ------------------ | ------------------- |
+| `item.title`       | Content title       |
+| `item.description` | Content description |
+| `item.date`        | Publication date    |
+| `item.path`        | URL path            |
+| `item.slug`        | URL slug            |
+
+## Template Override
+
+Set `template` in frontmatter to use a custom template:
+
+```yaml
+---
+title: Special Page
+template: special
+---
+```
+
+This uses `templates/page/special.html` instead of the default.