docs(readme): add macOS Homebrew install + pre-flight checks; clarify FFmpeg versions; fix ToC/typo

[Else00]

Adds a dedicated “Prerequisites Installation on macOS (Homebrew)” section with a minimal, copy-pasteable flow:

• install deps via Homebrew
• session-only exports (PKG_CONFIG_PATH, OPENCV_INCLUDE_PATHS, OPENCV_LINK_PATHS)
• quick pkg-config checks
• cargo install/update
• optional one-off fallback with ffmpeg@7

Also:

• clarifies FFmpeg support to “Tested with FFmpeg 6.1 (Linux) and 7.x (macOS/Homebrew)”
• fixes ToC Linux anchor (#prerequisites-installation-on-ubuntu-linux)
• replaces xcode-select --install with a pre-check to avoid non-zero exit codes
• fixes playback commands typo for 0-9.