Restructure docs

[dineshyv]

Rendered docs at: https://llama-stack.readthedocs.io/en/doc-simplify/

[ashwinb]

High level comments:

• The title for the section could be Start the inference server to make it more generic and internally we say "Ollama is the quickest way; but here are all the other ways"
• nit: "llama stack" should be "Llama Stack"
• nit: "Sample app code" could be "Your first RAG agent" ?

[dineshyv]

done