Basically, we need to update all the documentation from related to: #1702

We should include:

Selector concepts: How selectors work and the source vs flag set decoupling
Selector syntax: flagSetId=app vs source=config.json patterns and precedence rules
Metadata reflection: How selector information is echoed back in responses for debugging
Migration guidance: Transitioning from source-based to flag-set-based selectors
Provider configuration: Using selectors in SDKs and the Flagd-Selector header
Troubleshooting: Common selector issues and debugging steps