linkerd.io/reference could redirect to latest reference docs

[cratelyn]

as of writing, the latest reference documentation lives at https://linkerd.io/2.16/reference/. other versions' documentation can be accessed at :{version}/reference/.

it would be nice if https://linkerd.io/reference/ redirected clients to the latest reference, much like how https://linkerd.io/docs redirects to https://linkerd.io/2.16/overview/.