Add a custom 404 page for readthedocs

[tennlee]

It's possible to add a custom 404 page for scores on readthedocs. This would be worth doing, as there are some search results to nonexistent bits of documentation, or if follows a bad or dead link for any reason. Adding the custom 404 would allow a redirect to latest or develop documentation, so that it's clear there is at least documentation there to be read.