[Mistral, Mixtral] Improve docs

[NielsRogge]

What does this PR do?

This PR improves the docs of Mistral and Mixtral, by including:

• explaining the difference between base models vs instruction tuned ones, including the newer v2 checkpoint of Mistral-7B
• use of chat templates
• quantization tips and tricks
• memory requirements
• resources such as the Alignment Handbook by HF

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[ArthurZucker]

Thanks for keeping this up to date!