x/website/cmd/golangorg: redirect golang.org/x/pkgsite to pkg.go.dev/golang.org/x/pkgsite

[julieqiu]

Pkg.go.dev will be open-sourced with the module path golang.org/x/pkgsite. It would be great if https://golang.org/x/pkgsite redirected to https://pkg.go.dev/golang.org/x/pkgsite.

Related issues: #36106 #36747

/cc @dmitshur

[gopherbot]

Change https://golang.org/cl/234677 mentions this issue: repos: add API for opting-in to use pkg.go.dev for documentation

[gopherbot]

Change https://golang.org/cl/234678 mentions this issue: cmd/golangorg: add golang.org/x/pkgsite repository

[gopherbot]

Change https://golang.org/cl/289700 mentions this issue: repos: remove unused Repo.UsePkgGoDev