chore: add current feature flags to documentation

[caalador]

touches vaadin/flow#14890

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AI Language Review

In the file Feature Flags, several additions have been made to list new experimental features under different categories such as "Flow Feature Flags", "Flow Components Feature Flags", and "Hilla Feature Flags". The descriptions for each new feature are concise, though the following suggestions could improve consistency and clarity:

1. In the "Flow Feature Flags" section:

   • The description for collaborationEngineBackend currently reads, "Collaboration Kit backend for clustering support." Consider rephrasing to ensure consistency in sentence structure across all feature descriptions. For example, "Enables the Collaboration Kit backend for clustering support."

2. In the "Flow Components Feature Flags" section:

   • The masterDetailLayoutComponent is listed again here, but it seems redundant since it's already defined in the "Flow Feature Flags" section. Verify if this is intentional or needs consolidation to avoid repetition.

Ensure all feature descriptions follow a similar format and provide a clear and informative summary of the feature's functionality.