docs: Add pretty download URLs

This gives nicer download URLs for OPA binaries via:

https://openpolicyagent.org/downloads/<version>/<binary>

Under the hood we just use Netlify to rewrite the url to the normal
github and s3 locations.

With this you can also use:

https://openpolicyagent.org/downloads/edge/<binary>

and

https://openpolicyagent.org/downloads/latest/<binary>

For quick links to the current master and latest release binaries.

Fixes: #1771
Signed-off-by: Patrick East <east.patrick@gmail.com>