Setup a redirect from /colours/ to /colors/

As a result of https://github.com/ember-learn/ember-styleguide/pull/315 we have updated the spelling of `color` in the application. This is all fine for the content but if we update any URL we should make sure that there is a redirect in place in all cases 👍

This task will be complete when https://ember-styleguide.netlify.app/concepts/colours/ redirects to https://ember-styleguide.netlify.app/concepts/colors/

Note: this implementation may not currently be implemented in `field-guide` 🤔 but it should be reasonably easy to port the same redirect code from Guidemaker to Field Guide

Start by checking how redirects are implemented in Guidemaker, then inspect the corresponding Field Guide setup mentioned in the issue. Reproduce the request at /concepts/colours/ and compare it with /concepts/colors/. Done means the former redirects to the latter on the deployed site.

Written by the indexing model from the issue text.