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 3.14.2
• Simplify guide from 454 to ~256 lines
• Add sections for virtual environments and multiple Python versions
• Remove outdated options (Anaconda, Chocolatey, Homebrew, building from source)
• Add link to Talk Python episode #476 about uv
• Create change-log.md to track future releases
• Tag release as python-2025-guide