doc: replace all of gh-pages w/ a single 404.md page with links (#6437) Fixes: https://github.com/googleapis/google-cloud-cpp/issues/6085 Removes all content in https://github.com/googleapis/google-cloud-cpp/tree/gh-pages Replaces it with a single 404.md file that tells users that the content has moved and gives them links to the developer docs. I used a 404 page so that it will work regardless of any inbound link that a user may have used. Here's a screenshot of the 404 page that I created from my fork:  #1