feat(docs): Add IPEX-LLM section in LLM backend guide (for local model deployment on Intel CPU/GPU)

[shane-huang]

This PR adds IPEX-LLM as a new local LLM backend to the LLM Backends Page

IPEX-LLM is a library which allows users to run LLMs locally on Intel CPU and GPU with very low latency. With IPEX-LLM, user can leverage even the integrated GPU of low-cost PCs to run LLMs with a smooth experience.

PrivateGPT can successfully connect to LLMs deployed with IPEX-LLM, and there's a detailed quickstart guide with a demo video at IPEX-LLM docs website (see Run PrivateGPT with IPEX-LLM Quickstart ).

Any suggestions on document styles or contents, please kindly let me know.