Modernize Python installation guide for 2025 with uv

- Replace 10 different installation methods with unified uv approach
- Update target Python version from 3.12 to 3.14
- Add sections for virtual environments and multiple Python versions
- Remove outdated options (Anaconda, Chocolatey, Homebrew, building from source)
- Reduce guide from 454 lines to ~180 lines

Author: mikeckennedy