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

Completes AI audit (4 layers passed) and documentation gap analysis.
All 39 tests pass, docs site builds successfully.

docs/content/features/css.md

@@ -0,0 +1,48 @@
+---
+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)