Add a more detailed user manual (initial version)

[liu-cong]

What would you like to be added:

In addition to our awesome get started guide, we should have a more detailed user manual doc for more advanced configuration. For the initial cut, we should include the following:

• Model server compatibility For each release changing the model server protocol, document the oldest compatible version of each supported model server #482
• Routing algorithm config knobs
• How to configure a compatible model server other than the default (vllm)

Why is this needed:

[liu-cong]

/assign I can draft the initial version