docs: Refine the existing docs and create a new doc outline

[Sherlock113]

The existing BentoML docs need to be refined in terms of language, structure, tone, content, and more. It is better to get started with the Overview and Quickstarts section. During the process, new docs will be added as well.

A proposed outline (will be updated as more docs being added):

Overview

• [x] What is BentoML
• [x] Ecosystem

Quickstarts

• [x] Install BentoML
• [x] A Transformer model deployment doc
• [x] An LLM deployment doc using OpenLLM
• [x] An Iris deployment doc
• [x] Yolo deployment

Concepts

• [ ] Models
• [ ] Services and APIs
• [ ] Bentos
• [ ] Runners
• [ ] Deployments

Tutorials

Integrations

Reference

[Sherlock113]

The Overview and Quickstarts sections were done.