Add pm exec instructions to docs

[manuel3108]

In #214 we discussed that some exec commands like the deno one are not straightforward.

Based on this comment, here are a few commands that we could display

• NPM : npx sv create
• PNPM : pnpm dlx sv create
• Yarn : yarn dlx sv create
• Bun : bunx sv create
• Deno : deno run npm:sv create

Users should mostly know how to use this, but some newcomers might not. Adding it to the docs site once published seems reasonable. We could maybe display them in tabs as many other docs support. But we will need to see what svelte.dev supports.