docs(features): add feeds and css minification documentation

- Add Atom feed generation docs (feeds.md)
- Add CSS minification docs (css.md)
- Fix em dash usage in css.md for natural prose

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docs/content/features/css.md

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+---
+title: CSS Minification
+description: Automatic CSS optimization at build time
+weight: 7
+---
+
+# CSS Minification
+
+sukr automatically minifies CSS files in your static directory during the build.
+
+## How It Works
+
+When copying files from `static/` to your output directory:
+
+1. CSS files (`.css` extension) are processed with lightningcss
+2. Whitespace, comments, and redundant rules are removed
+3. Identical selectors are merged
+4. Other static files are copied unchanged
+
+## Build Output
+
+You'll see minification progress during builds:
+
+```text
+minifying: static/style.css → public/style.css (2048 → 1234 bytes)
+copying: static/logo.svg → public/logo.svg
+```
+
+## No Configuration
+
+Minification is always on. There's no setting to disable it, so if you need the original CSS, check your source files.
+
+## Error Handling
+
+If CSS parsing fails (malformed input), sukr preserves the original file content instead of failing the build. Check your terminal for warnings.
+
+## What Gets Minified
+
+- Whitespace and newlines removed
+- Comments stripped
+- Selector merging (`.a { color: red } .b { color: red }` → `.a, .b { color: red }`)
+- Vendor prefix optimization
+
+## What Doesn't Change
+
+- CSS variable names
+- Class and ID selectors
+- Relative paths (images, fonts)

docs/content/features/feeds.md

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+---
+title: Atom Feeds
+description: Build-time feed generation for blog posts
+weight: 6
+---
+
+# Atom Feeds
+
+sukr generates an Atom 1.0 feed for blog posts at build time.
+
+## Output
+
+After building, you'll find `feed.xml` in your output directory:
+
+```text
+public/
+├── index.html
+├── feed.xml  ← Atom feed
+└── blog/
+    └── ...
+```
+
+## Feed Contents
+
+The feed includes:
+
+- Site title and author from `site.toml`
+- Self-referencing links (required by Atom spec)
+- Entry for each content item in `blog/` section
+- Post title, URL, date, and description
+
+## Auto-generation
+
+Feed generation happens automatically when any content exists in a section with `section_type: blog`. No configuration required.
+
+Posts are sorted by date (newest first), matching the blog section ordering.
+
+## Linking to the Feed
+
+Add a link in your `base.html` template:
+
+```html
+<link
+  rel="alternate"
+  type="application/atom+xml"
+  title="{{ config.title }} Feed"
+  href="{{ prefix }}/feed.xml"
+/>
+```
+
+## Date Format
+
+Post dates in frontmatter should use `YYYY-MM-DD` format:
+
+```yaml
+---
+title: My Post
+date: 2024-01-15
+---
+```
+
+The feed converts this to RFC 3339 format required by Atom.
+
+## Validation
+
+Test your feed with the [W3C Feed Validator](https://validator.w3.org/feed/).