[Docs] Add Azure/AKS deployment guide

## Proposal

Add an Azure AKS deployment guide to the installation documentation,
similar to the existing AWS and GCP guides.

## Scope
- AKS cluster setup with GPU node pool (NC-series A100)
- NVIDIA GPU operator installation on AKS
- Envoy Gateway configuration on AKS
- AIBrix Helm installation and validation
- End-to-end example deploying a model with vLLM

## Motivation
Azure/AKS is the only major cloud provider without a deployment guide.
AIBrix docs state it's compatible with vanilla Kubernetes, but there's
no validated AKS-specific walkthrough.

I'm happy to implement this and submit a PR once there's agreement
on scope.