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

.github/instructions/features.instructions.md

@@ -0,0 +1,26 @@
+---
+description: "Guidance for writing and formatting the `docs/features.md` file."
+applyTo: "docs/features.md"
+---
+
+# Features Documentation Guidelines
+
+When creating or updating the `docs/features.md` file, please adhere to the following guidelines to ensure clarity and consistency:
+
+## Structure
+
+   - This document should provide a short, to the point overview of each feature. It is used for marketing of the project. A quick read of what the feature is and why it matters. It is the "elevator pitch" for each feature.
+   - Each feature should have its own section with a clear heading.
+    - Use bullet points or numbered lists to break down complex information.
+    - Include relevant links to other documentation or resources for further reading.
+    - Use consistent formatting for headings, subheadings, and text styles throughout the document.
+    - Avoid overly technical jargon; the document should be accessible to a broad audience. Keep novice users in mind.
+    - This is not the place for deep technical details or implementation specifics. Keep those for individual feature docs.
+
+## Content
+    - Start with a brief summary of the feature.
+    - Explain the purpose and benefits of the feature.
+    - Keep
+    - Ensure accuracy and up-to-date information.
+
+## Review