docs: Update README and installation guide for LiteLLM backend addition

[TPLin22]

Description

We support using LiteLLM as LLM-Backend to use various model more flexibly.

Motivation and Context

How Has This Been Tested?

• [ ] If you are adding a new feature, test on your own test scripts.

Screenshots of Test Results (if appropriate):

1. Your own tests:

Types of changes

• [ ] Fix bugs
• [ ] Add new feature
• [x] Update documentation

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📚 Documentation preview 📚: https://RDAgent--703.org.readthedocs.build/en/703/