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/sections.md

@@ -0,0 +1,71 @@
+---
+title: Sections
+description: Automatic section discovery and processing
+weight: 2
+---
+
+# Sections
+
+sukr automatically discovers sections from your content directory structure.
+
+## What is a Section?
+
+A section is any directory under `content/` that contains an `_index.md` file:
+
+```
+content/
+├── _index.md           # Homepage (not a section)
+├── about.md            # Standalone page
+├── blog/               # ← This is a section
+│   ├── _index.md       # Section index
+│   └── my-post.md      # Section content
+└── projects/           # ← This is also a section
+    ├── _index.md
+    └── project-a.md
+```
+
+## Section Discovery
+
+sukr automatically:
+
+1. Scans `content/` for directories with `_index.md`
+2. Collects all `.md` files in that directory (excluding `_index.md`)
+3. Renders the section index template with the items
+4. Renders individual content pages (for blog-type sections)
+
+## Section Types
+
+The section type determines which template is used. It's resolved in order:
+
+1. **Frontmatter override**: `section_type: blog` in `_index.md`
+2. **Directory name**: `content/blog/` → type `blog`
+
+### Built-in Section Types
+
+| Type       | Behavior                                               |
+| ---------- | ------------------------------------------------------ |
+| `blog`     | Sorts by date (newest first), renders individual posts |
+| `projects` | Sorts by weight, card-style listing                    |
+| _(other)_  | Sorts by weight, uses default template                 |
+
+## Section Frontmatter
+
+In `_index.md`:
+
+```yaml
+---
+title: My Blog
+description: Thoughts and tutorials
+section_type: blog # Optional, defaults to directory name
+weight: 1 # Nav order
+---
+```
+
+## Adding a New Section
+
+1. Create directory: `content/recipes/`
+2. Create index: `content/recipes/_index.md`
+3. Add content: `content/recipes/pasta.md`
+4. Optionally create template: `templates/section/recipes.html`
+
+That's it. sukr handles the rest.