feat: Prepare for NuGet package publishing

Updated project metadata and added publishing guides:
- Updated Databento.Client.csproj with v3.0.5-beta metadata
- Updated Databento.Interop.csproj with v3.0.5-beta metadata
- Added package description, tags, and release notes
- Configured README.md inclusion in NuGet package
- Added comprehensive NUGET_PUBLISHING_GUIDE.md with:
  * Step-by-step publishing instructions
  * User installation and usage examples
  * Troubleshooting section
  * Update procedures
- Added NUGET_QUICK_START.md for quick reference

Package is now ready for publishing to NuGet.org

Author: Serha