Remove old versions of the docs from being discoverable on websearches

[George Datseris]

Following https://discourse.julialang.org/t/ann-documenter-0-27/62726#warnings-and-seo-for-outdated-docs-1 , in the local clone of the repo, on the gh-pages branch, do:

using DocumenterTools
OutdatedWarning.generate("/path/to/your/docs")

where the path is given to the root directory of the repo. The one where your .git and /src directories would normally be. But on the gh-pages branch it is a bunch of vX.Y directories.