docs: update CONTRIBUTING to use npm/pnpm (remove yarn)

[White-Devil347]

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[White-Devil347]

docs: update CONTRIBUTING to use npm/pnpm (remove yarn)

[Sean-Kenneth-Doherty]

I reviewed this CONTRIBUTING update and it looks directionally good.

Checks run:

• git diff --check main...HEAD passes
• compared the commands against the current package.json scripts (dev and build) and the README, which already documents npm/pnpm instead of yarn

The important corrections are right: yarn start and port 3000 are stale for this Astro app, while npm run dev / pnpm dev and port 4321 match the current project.

Small non-blocking polish: I would lowercase “Dependencies” in “install the required Dependencies” while this section is being touched.