refactor(readme): clear rewriting of readme

[Akinator31]

This pull request significantly restructures the documentation for the Rustmail project to improve clarity, usability, and maintainability. The main README.md is simplified, with detailed sections about commands and setup moved to new dedicated documentation files. Additionally, configuration and feature documentation are updated to reflect recent changes, such as the addition of reminders.

Key documentation and configuration changes:

Documentation Restructuring:

• The README.md is streamlined to provide a high-level overview, warnings, and links to new, focused documentation files for setup and commands. Most detailed usage, configuration, and feature explanations are now in docs/setup.md and docs/commands.md.

New Documentation Files:

• Added docs/commands.md with comprehensive tables and explanations for both slash and text commands, including usage examples and descriptions for each command.
• Added docs/setup.md with step-by-step instructions for installing, configuring, and running Rustmail, including links to the example configuration and release downloads.

Configuration Updates:

• Updated config.example.toml to add a [reminders] section with an embed_color setting, reflecting new or upcoming reminder features.

Support and Contribution Guidance:

• The README.md now includes a clear support section directing users to the Discord server for help, and retains contribution and warning notices.

These changes make the documentation more modular and user-friendly, and prepare the project for future feature expansion.