Add Sphinx docs site with example previews; fix dropped PDF background

[connorferster]

Summary

Adds a full documentation site and a runnable Examples gallery with rendered-PDF previews, and fixes a regression that silently dropped PDF backgrounds.

Documentation

• New Sphinx site under docs/ using the Shibuya theme (light) with MyST markdown: landing page, installation, quickstart, guides (document structure, configuration, variables, PDF backgrounds), and reference (blocks, CLI, fonts).
• Examples gallery (docs/examples.md) that pulls each example's YAML via literalinclude and shows the actual rendered-PDF previews (generated with ym convert → PyMuPDF), so docs stay in sync with Examples/.
• Click-to-enlarge lightbox for previews — accessible, dependency-free (× / backdrop / Esc to close).
• Refreshed the top-level README with YMPrint branding, logo, and summary.

Fix

• context_builder stored the doctemplate config double-wrapped as {"_doc": {"_doc": {...}}}, so load_pdf_backgrounds and the first-page check read the wrapper and silently dropped background / first-page. Now stores combined_doctemplate directly so ['yaml']['_doc'] is the real doc config again.
• Regenerated all example PDFs with the fix (the PDF-backgrounds example renders its template + populated form fields again).

Build

Zero-warning build:

uv run --with-requirements docs/requirements.txt \
    sphinx-build -b html docs docs/_build/html

Commits

• Add Sphinx docs site + example previews; fix dropped PDF background
• Add click-to-enlarge lightbox to Examples previews

🤖 Generated with Claude Code