Replace DevDocs references with GJS-Docs

Update a bunch of devdocs.io / DevDocs references to use gjs-docs.gnome.org / GJS-Docs instead: In the documentation, in the site news, in the OpenSearch template, in the 404 and 500 error page templates, and in the default config.

I also commented out the entire section about submitting new documentation alias suggestions, because our doc names are short enough to make that a pointless waste of everyone's time.