docs: list `pnpx` as alias for `pnpm dlx`

[benmccann]

Other docs have aliases listed. E.g. https://pnpm.io/cli/update has up and upgrade

I wrote this one out in a bit longer format so that no one would think you should run pnpm pnpx

[stackblitz[bot]]

Run & review this pull request in StackBlitz Codeflow.

[vercel[bot]]

@benmccann is attempting to deploy a commit to the pnpm Team on Vercel.

A member of the Team first needs to authorize it.