Fix quickstart npm install to also fetch the Lightpanda binary

[cdebled]

Summary

• Installing @lightpanda/browser via npm only gets the JS library, not the browser binary, so npm install --save @lightpanda/browser alone leaves lightpanda.serve() with nothing to spawn. Added npx @lightpanda/browser install as a second step in the npm and pnpm tabs.
• Preferred npx @lightpanda/browser install over the curl one-liner (pkg.lightpanda.io/install.sh) because it uses the get.lightpanda.io/versions.json endpoint, more reliable than api.github.com, which the one-liner (and the package's own broken postinstall script) depends on and which is prone to rate-limiting.
• Dropped the yarn tab: @lightpanda/browser's package.json declares "engines": {"yarn": "4"}, so yarn add hard-fails on Yarn Classic (v1), the default most people have.
• Refined the intro sentence for the install step.