x/website: use all gopher images on the website

[fzipp]

Some gophers did not survive various website reorganisations over the time and are no longer featured on the website:

• https://golang.org/doc/gopher/ref.png
• https://golang.org/doc/gopher/run.png

This gopher is also endangered:

• https://golang.org/doc/gopher/pkg.png
  It's currently featured on https://golang.org/pkg/ but the redirect to https://pkg.go.dev/std is imminent (x/website: redirect golang.org/pkg and golang.org/cmd to pkg.go.dev #44356). Maybe it can find a new home over there?

I think the 'ref' gopher belongs in this section: https://golang.org/doc/#references

[dmitshur]

We should certainly not delete the gopher images without reason, but I think it's okay for some of them not to featured on the website. What's important is they're available to be featured when there is a need or desire. Until then, some gophers can rest in the tunnels.

I'll close this because I don't think we need a tracking issue for this, but thanks for thinking of the gophers!