Fix #29: document and verify custom font support

[bitomule]

Summary

• Document how to use custom fonts in standard YAML content screenshots with font_family
• Resolve relative .ttf, .otf, and .ttc font paths from the YAML config directory before rendering
• Document HTML template font loading with assets and @font-face
• Add YAML and HTML tests that copy a real local font file, render/generate screenshots, and verify asset staging

Closes #29

Tests

• .venv/bin/pytest tests/test_generator.py tests/test_html_renderer.py -k "font or custom_font or template_custom_font or exposes_font" -q
• .venv/bin/pytest tests/test_generator.py tests/test_html_renderer.py -q
• .venv/bin/pytest -q