Concourse site should allow basic redirects

[jama22]

With the handover to the new site, we made some changes to the names of specific pages and the layout of information. Since Concourse is a 3+ year old project there are a lot of links floating around; it would be nice if we can somehow re-route the old URLs to the new URLs.

An easy example of this is the Downloads page https://github.com/concourse/concourse/issues/2159 Another could be smartly re-directing fly command URLs to the new URLs