Update links to point to new help docs

@almonk Would you kindly? :alien: 
hey @jacobpgn - thanks for this - unfortunately though it's failing the linkchecker test script that gets run automagically on CircleCI...

/payment-timings already redirects to "https://help.gocardless.com/customer/portal/articles/1563666" - so the link checker now gives us a big 'fail' if we point at our own redirect - we should be pointing straight at the article itself.

I can fix this if you like, or if you want to u can go for it, and I can review?