feat: Multi-site support

[rochacbruno]

Summary

• Implements multi-site support allowing subsites within content directories
• Each subsite can have independent configuration, theme, and search index
• Subsites are accessible at their own URL paths (e.g., /site1/)

Changes

• Modified site generation to detect and process directories with site.yaml files
• Updated server routing to handle subsite directory requests
• Fixed various compilation issues with proper type handling
• Added example subsite at example/content/site1/
• Created documentation for the multi-site feature

Test plan

• Build the project: cargo build
• Generate site with subsite: cargo run example
• Verify subsite files are generated at example/site/site1/
• Start server: cargo run example --serve
• Access main site at http://localhost:8000/
• Access subsite at http://localhost:8000/site1/
• Verify subsite has independent theme (theme_template)
• Check search functionality is independent
• Verify RSS feeds are generated for subsite

Closes #329