Add redirect from execution-specs gh pages to steel-website docs/spec and delete `gh-pages.yaml`

Once the mkdocs HTML site has gone live at steel.ethereum.foundation/docs/ we should redirect from the existing rendered spec docs to the new location and delete the now defunct gh-pages workflow.

We'll probably have to figure how to ensure that the redirect goes to the latest default branch spec at steel.ethereum.foundation/docs/, but it might have been solved in the meantime.