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

docs/content/_index.md

@@ -635,13 +635,13 @@ To get started download an OPA binary for your platform from GitHub releases:
 On macOS (64-bit):
 
 ```shell
-curl -L -o opa https://github.com/open-policy-agent/opa/releases/download/{{< current_version >}}/opa_darwin_amd64
+curl -L -o opa https://openpolicyagent.org/downloads/{{< current_version >}}/opa_darwin_amd64
 ```
 
 On Linux (64-bit):
 
 ```shell
-curl -L -o opa https://github.com/open-policy-agent/opa/releases/download/{{< current_version >}}/opa_linux_amd64
+curl -L -o opa https://openpolicyagent.org/downloads/{{< current_version >}}/opa_linux_amd64
 ```
 
 > Windows users can obtain the OPA executable from [GitHub

docs/website/layouts/index.redirects

@@ -25,3 +25,8 @@
 # Legacy git book redirects
 /docs/{{ . }}.html /docs/latest/{{ . }}
 {{- end }}
+
+# Download URLs
+/downloads/edge/*       https://opa-releases.s3.amazonaws.com/edge/:splat 200
+/downloads/latest/*     https://github.com/open-policy-agent/opa/releases/download/{{ $latest }}/:splat 200
+/downloads/*            https://github.com/open-policy-agent/opa/releases/download/:splat 200