x/website: Support gotip in playground

[icholy]

What did you do?

Navigated to https://go.dev/gotipplay/

What did you expect to see?

Something equivalent to: https://gotipplay.golang.org/

What did you see instead?

open gotipplay: file does not exist

[seankhliao]

This is already in progress in CL 366056

[icholy]

@seankhliao is there another issue tracking that work?