Add an automatic check that URL redirects work

[StevenMaude]

In #1064, I found that some redirects I added weren't working as expected. I'm still not sure why that is yet.

Depending on the reason for it (are we encountering a limit? or is it something more subtle?) we might want to add a scheduled check that:

• at minimum, the URL being redirected gives the redirect code specified
• and, maybe even that the URL being redirected gets redirected to the correct destination

[StevenMaude]

As a semi-automated process:

test $(cut -f1 -d' ' _redirects | awk '{print "https://3b9cf57d.opensafely-docs.pages.dev"$0}' | xargs -n1 curl -o /dev/null --silent --head --write-out '%{url_effective} '%{http_code}' \n' | grep "404" | wc -l) -eq 0

displays the URL and status code for each redirect, assuming a 404 catches every issue (which it possibly doesn't).

The better approach would be to compare the status code from the _redirects file with that from cURL.

[StevenMaude]

This was completed in #1094.

See also #1207, where using a MkDocs plugin for redirects is proposed.