docs: Add uv installation guide (docs/install/uv.md) and inline callouts

[mnriem]

Problem

Installation instructions use uv tool install / uvx as the first command users run, but neither the README nor docs/installation.md explains what uv is or how to install it. Users on all platforms get command not found: uv on first attempt.

Closes #1351
Closes #1357

Proposed fix

1. Create docs/install/uv.md — a dedicated page covering:

   • What uv is (fast Python package manager by Astral)
   • Installation per platform: standalone installer (curl/irm), Homebrew, WinGet/Scoop
   • Verification (uv --version)
   • Link to full Astral docs

2. Add inline callouts before the first uv command in:

   • README.md (Get Started section)
   • docs/installation.md (Prerequisites section)

   Something like: "Requires uv — install it first if you don't have it."

The docs/install/ directory already exists (empty), so this fits the intended structure.