Add dedicated docs section for managing the Octave executable

[blink1073]

Summary

• Adds a new "Managing the Octave Executable" section to README.rst, placed between "Supported Platforms" and "Troubleshooting"
• Documents the executable resolution order (config trait → OCTAVE_EXECUTABLE → octave on PATH → octave-cli on PATH → Flatpak fallback)
• Covers custom executable configuration via env var and octave_kernel_config.py
• Adds Windows-specific path guidance for Octave 5+
• Explains Flatpak/Snap auto-detection and the Snap-specific temp directory
• Describes startup validation behavior and points to python -m octave_kernel.check

Test plan

• Read through the new section and verify accuracy against kernel.py logic
• Confirm RST renders correctly (doc8 passes in pre-commit)