Redirect or unpublish old gh-pages

[janbrasna]

Can't remember where I found the link, but I eventually ended up looking at https://mozmeao.github.io/www-config/configs/ which is simply outdated. It hasn't been generated for ~5years and all the docs are being published to RTFD.org now…

The simple fix could be unpublishing the Pages from repo settings, deleting the environment/deployment, and removing gh-pages branch.

Or instead, I could draft a PR that stubs out the 2-3 .html pages in that branch with a meta redirection to the current docs, to keep the links working?