[doc] Enable Proxy Forwarding

The proxies and load balancers usually support `X-Forwarded` headers to send information altered or lost when a proxy is involved in the path of the request.
Jetty supports the ForwardedRequestCustomizer customizer to handle `X-Forwarded` headers.

Set the `customizer` attribute via the `web` element to enable the ForwardedRequestCustomizer customizer, ie:

```xml
<web bind="http://localhost:8161" path="web" customizer="org.eclipse.jetty.server.ForwardedRequestCustomizer">
<app url="activemq-branding" war="activemq-branding.war"/>
<app url="artemis-plugin" war="artemis-plugin.war"/>
<app url="console" war="console.war"/>
</web>
```