Gateway API - Add docs for traffic splitting example

[scseanchow]

Problem Statement

There don't exist docs on how end users can use weighted load balancing via Gateway API

Acceptance Criteria

• Tutorial style docs are added to inform the user how to use weight load balancing

[pmalek]

I consider this blocked for now until we agree on a new image to be used as the "echo service" so that we don't multiply work (first add traffic splitting examples using old images and then rework that into using the new image).