Update documentation for refactored bot

[ZenonEl]

Description

Documentation is outdated after major refactoring. Needs full update.

Tasks

• Update README.md — Docker setup, .NET 9, new features, removed MySQL
• Update docs/documentation/en/ — installation, configuration guides
• Update docs/documentation/ru/ — Russian versions
• Update appsettings.json.example with all current options and comments
• Update downloader-config examples if applicable
• Add CONTRIBUTING.md with dev setup instructions
• Update docs/adr/ with architecture decisions from refactoring