docs: rewrite features.md and add DNS challenge documentation

Complete documentation overhaul for DNS Challenge Support feature (PR #461):

Rewrite features.md as marketing overview (87% reduction: 1,952 → 249 lines)
Organize features into 8 logical categories with "Learn More" links
Add comprehensive dns-challenge.md with:
15+ supported DNS providers (Cloudflare, Route53, DigitalOcean, etc.)
Step-by-step setup guides
Provider-specific configuration
Manual DNS challenge workflow
Troubleshooting section
Create 18 feature documentation stub pages
Update README.md with DNS Challenge in Top Features section
Refs: #21, #461

docs/features/security-headers.md

@@ -0,0 +1,24 @@
+---
+title: HTTP Security Headers
+description: Automatic security headers including CSP, HSTS, and more
+---
+
+# HTTP Security Headers
+
+Modern browsers expect specific security headers to protect your users. Charon automatically adds industry-standard headers including Content-Security-Policy, Strict-Transport-Security, X-Frame-Options, and X-Content-Type-Options.
+
+## Overview
+
+<!-- TODO: Add detailed overview -->
+
+Coming soon.
+
+## Configuration
+
+<!-- TODO: Add configuration steps -->
+
+Coming soon.
+
+## Related
+
+- [Back to Features](../features.md)