Introduce `Versions` property in `EssentialsConfig` to hold version information.
Add `VersionData` class for Essentials and package versions, and `NugetVersion` class for individual package details.
Retain and document `SystemTemplateConfigs` class.
Refactor config handling and improve documentation
- Updated `PortalConfigReader.cs` to use constants for configuration keys, enhancing maintainability and readability. Improved error logging with `Debug.LogError`.
- Modified `ConfigReader.cs` to handle v2 configuration format, streamlining the loading process and avoiding redundant parsing.
- Added XML documentation comments to properties in `EssentialsConfig.cs`, improving code documentation. Initialized `Rooms` property in the constructor.
- Enhanced `SystemTemplateConfigs` class with XML documentation for better clarity on its properties.
