Enhance Setup Documentation with FFmpeg Installation Instructions

[artursharykin]

This pull request introduces an update to the setup documentation to include instructions for installing FFmpeg. This change ensures users will not encounter FileNotFoundErrors during the 'Obtain Tone Color Embedding' step in demo_part1.ipynb.

Key Changes:

1. Added step-by-step instructions to download and extract FFmpeg binaries into the virtual environment directory. This is critical for ensuring that all necessary binaries are available to the Python environment, which prevents common errors during the execution of audio processing tasks in the OpenVoice project.

Motivation:
Several users reported issues related to missing FFmpeg binaries when following the current installation documentation, leading to errors that impede progress. This update aims to provide a smoother setup experience and reduce the need for troubleshooting during initial setup.

Expected Impact:
By integrating FFmpeg installation into the setup guide:

1. New users will have a more seamless initial setup experience.
2. Support queries related to FFmpeg installation errors should decrease.
3. Users can more quickly move to utilizing OpenVoice's capabilities without manual configuration detours.

Please review the updated documentation steps and provide feedback or approval for merging these changes into the main branch.