feat: add comprehensive tests, examples, and documentation

[chaholl]

Summary

• Add examples directory with practical usage examples demonstrating PromptPack Python functionality
• Add comprehensive unit tests for multimodal, template, and validator functionality
• Update CI pipeline with Python 3.13 support, linting, and coverage reporting
• Add documentation for examples in Astro/Starlight docs site

Changes

Examples (examples/)

• basic_usage.py - Loading packs, creating templates, formatting prompts with variables and fragments
• tools_example.py - Tool conversion, handler binding, and LangChain integration
• validation_example.py - Output validation with banned words, length limits, and regex patterns
• packs/ - Example PromptPack JSON files (customer-support, sales-assistant)

Tests

• test_multimodal.py - 19 new tests for multimodal content conversion (images, audio, video)
• test_template.py - 11 new tests for model overrides, defaults, async support
• test_validators.py - 10 new tests for multiple validators, edge cases

CI Pipeline (.github/workflows/ci.yml)

• Add Python 3.13 to test matrix
• Add linting for examples directory
• Add coverage reporting with Codecov
• Add examples verification job

Documentation (docs/)

• Add Examples section with 4 detailed guides
• Update quickstart with validation examples
• Add Output Validation feature card to home page

Test plan

• All 131 tests pass (60 promptpack + 71 promptpack-langchain)
• Linting passes (ruff check and ruff format --check)
• Type checking passes (mypy)
• All examples run successfully
• Docs build successfully