Mention playwright install requirement for tests in contributing.md

[mbafford]

Add mention of needing to run playwright install if contributing/testing changes.

The pytest tests don't clearly state why they fail if playwright isn't installed.

tests/run_examples.sh does mention it, in the ouput, but I'd expect people to run pytest first.

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:books: Documentation preview :books:: https://shot-scraper--85.org.readthedocs.build/en/85/