RFC: move doc to separate repository with a new structure

[dciangot]

As many feedback received recently, we'd like to consider moving the documentation on a separate repository and reworking the doc for the upcoming 0.6.0 release.

The proposed structure is the following:

Overview
Requirements # Minimal and Optimal
Quickstart # local daemons + k8s
User Guide/
├─ Getting started/
│  ├─ Simple pod # annotations
├─ ML & data analysis tools/
│  ├─ KubeRay cluster
│  ├─ Dask # Native+on ray
│  ├─ MPI job
├─ Serving & inference/
│  ├─ KServe
│  ├─ vLLM
├─ Batch & pipelines/
│  ├─ KubeFlow
│  ├─ Kueue
├─ Interactive Environments/
│  ├─ JupyterHUB # Binder
Admin Guide/
├─ Full stack deployment/
├─ Resource provider deployment/
├─ Kubernetes admin deployment/
├─ Monitoring/
├─ AuthN/Z/
├─ Configurations
User stories/
Advanced/
├─ Service account token mount
├─ Full mesh network deployment
├─ Advanced AuthN/Z models/
├─ Custom Job scripts
Plugins/
Develop/
├─ OpenAPI specs
├─ Developers environment
├─ SDKs/
Troubleshooting & FAQ
Known Limitations & Roadmap
Community # links and Governance included

Please take some time to review it and suggest any improvement!