Hello there! First real docs PR for uv.

1. I expect this will be rewritten a gazillion times to have a consistent tone with the rest of the docs, despite me trying to stick to it as best as I could. Feel free to edit!
2. I went super on the verbose mode, while also providing a callout with a TLDR on top. Scrap anything you feel it's redundant!
3. I placed the guide under integrations since Charlie added the FastAPI integration there.

Summary

Addresses #5945

Test Plan

I just looked at the docs on the dev server of mkdocs if it looked nice.

I could not test the commands that I wrote work outside of macOS. If someone among contributors has a Windows/Linux laptop, it should be enough, even for the GPU-supported versions: I expect the installation will just break once torch checks for CUDA (perhaps even at runtime).