Add automatic API documentation generation with Sphinx

[dreamlessx]

Description

Set up Sphinx autodoc to generate API reference documentation from docstrings.

Tasks

1. Create docs/ directory with Sphinx configuration
2. Add docs/conf.py with autodoc, napoleon (Google-style docstrings), and intersphinx
3. Create docs/api/ with one .rst file per module
4. Add docs/Makefile for building HTML docs
5. Add GitHub Actions job to build docs on PR and deploy to GitHub Pages on merge

Modules to document

• landmarkdiff.landmarks — face detection and landmark extraction
• landmarkdiff.manipulation — procedure-specific deformation
• landmarkdiff.conditioning — ControlNet conditioning generation
• landmarkdiff.evaluation — metrics computation
• landmarkdiff.api_client — REST API client
• landmarkdiff.synthetic — training pair generation

Acceptance criteria

• docs/conf.py configured with autodoc + napoleon
• make html builds without warnings
• At least 6 core modules documented
• CI job validates docs build on every PR