Docs: Add Envoy Gateway User Guide

[Xunzhuo]

/kind feature

Describe the solution you'd like

add user-facing docs via kserve/kserve#3952

Links to the design documents:
[Optional, start with the short-form RFC template to outline your ideas and get early feedback.]
[Required, use the longer-form design doc template to specify and discuss your design in more detail]

[sivanantha321]

@Xunzhuo Installation guide and migration guide with example is already available in the latest docs. If this docs needs any improvement, please create an issue in the kserve website repo.