docs: improve getting started installation docs

[peaklabs-dev]

Changes

• When using Svelte with package managers other than npm the installation is a bit cumbersome, as we have to translate each command ourselves. This PR adds commands for yarn, pnpm and bun.
• When using Svelte with something like https://inertiajs.com/ the installation is a bit difficult, since we must gather commands from various sources. To make this much easier the Getting Started guide now provides instructions for installing Svelte directly into an existing project with commands for all packages managers.
• The svelte-check package for tooling is also valuable (CI checks), so I added the commands to install it as well.

[changeset-bot]

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Latest commit: 8caa4b5

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-17365-svelte.vercel.app/