Docs: Add Envoy Gateway User Guide

/kind feature

**Describe the solution you'd like**

add user-facing docs via https://github.com/kserve/kserve/pull/3952

**Links to the design documents:**
[Optional, start with the short-form [RFC template](https://docs.google.com/document/d/1UcBeLfZ_JMGpVrPJmYtEIVH_9Y4U3AEKQdq_IKuOMrU) to outline your ideas and get early feedback.]
[Required, use the longer-form [design doc template](https://docs.google.com/document/d/1Mtoui_PP2a9N59NjYHnsvrdJ8t2iKFwIJAx1zRO_I1c) to specify and discuss your design in more detail]

Start by reading the KServe pull request linked in the issue to understand the Envoy Gateway documentation requested here. Then inspect the website's existing user documentation structure and identify where the guide belongs. Done means a complete, user-facing Envoy Gateway guide is added and integrated consistently with the site's documentation.

Written by the indexing model from the issue text.