Complete Playwright visual source: add optional dependency and documentation

[jmjava]

Summary

Issue #16 requested adding type: playwright as a visual source for browser-based capture. The implementation is already complete across the codebase. This PR closes the remaining gaps: an explicit optional dependency and complete documentation.

What was already implemented

• PlaywrightRunner class in playwright_runner.py - runs external capture scripts
• docgen playwright CLI command for standalone capture
• type: playwright dispatch in compose.py for pipeline integration
• Configuration block (playwright:) in docgen.yaml schema
• README section documenting type: playwright visual map entries and script contract
• Test coverage in test_playwright_runner.py and test_compose.py

What this PR adds

• playwright optional dependency in pyproject.toml (pip install docgen[playwright])
• Features section in README now lists Playwright browser capture
• System dependencies section now includes Playwright with install instructions
• Formatting fix: missing blank line before VHS safety section

Acceptance criteria from issue #16

Criteria	Status
docgen playwright --script my_script.py --url http://localhost:3300 produces an MP4	Done (CLI + runner)
type: playwright segments compose correctly with narration audio	Done (compose.py)
Headless mode works on Ubuntu (CI-compatible)	Done (uses headless Chromium)
Documentation updated with Playwright visual source examples	Done (README)

Closes #16