Add redirect for /guidance

Requests to www.gov.uk/guidance should respond with something other than a 404 in case people request it via URL hacking from a manual page.

Until we build a better option, the list of guidance publications published in Whitehall is the best we have.

Related to https://trello.com/c/0jDZiBik/134-hmrc-manual-and-manuals-deploy-manuals-frontend-to-production-2