[Docs] Revise Vizro-AI docs to align with style guide

Description

Now ready for review. You can see the built docs here: https://vizro--381.org.readthedocs.build/projects/vizro-ai/en/381/

I have made changes that should be familiar given the set I've made on vizro-core to follow the style guide. For example, I've change capitalisation in headers to sentence case.

I also rationalised the installation guide to align it more closely to the one for vizro-core and I converted a page to FAQs since it was better titled to answer common queries. The LLM API key content moves into the installation guide since, for now at least, it's covering just OpenAI and doesn't need to keep redirecting the reader around the docs to different places. I think getting the .env file set up is probably the most confusing thing for novice users who are unfamiliar with API keys and environment settings...we need to think about about a video at some point.

I moved some text around on the index page to remove the most "promotional" copy (that can go into the README if we need it) and keep it short and useful. The box navigation is now above the fold.

Screenshot

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mckinsey / vizro