docs: add docs for selector, metadata

[toddbaert]

This PR:

• adds documentation for our new selector features, added mostly in feat: multi-project support via selectors and flagSetId namespacing #1702
  • conceptual docs
  • practical usage docs
  • additions to troubleshooting
  • migration guide

Fixes: #1828

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[gemini-code-assist]

Summary of Changes

Hello @toddbaert, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the documentation for flagd's selector features, which allow for more granular and efficient filtering of flag configurations. The updates cover the core concepts of selectors and flag sets, provide practical guidance for their usage, and include a detailed migration path for existing users. Additionally, it addresses potential troubleshooting scenarios and clarifies the syntax and precedence of selectors across various flagd components.

Highlights

• New Selector Concepts Documentation: Introduced a new conceptual document explaining flagd's selector features, detailing flag sources, flag sets, metadata reflection, and their decoupling for more flexible flag management.
• Migration Guide for Flag Sets: Added a comprehensive guide to assist users in transitioning from legacy source-based selector patterns to the new, recommended flag set-based patterns, ensuring backward compatibility.
• Detailed Selector Syntax Reference: Provided a technical specification for selector syntax, including supported keys like flagSetId and source, precedence rules, and how metadata reflection works.
• Updated Provider and Sync Configuration Docs: Enhanced existing documentation for provider and sync configurations to reflect the new selector capabilities, including usage examples and precedence for different selector input methods.
• Troubleshooting for Selector Issues: Expanded the troubleshooting guide with a dedicated section for common selector-related problems, offering debugging steps for issues like no flags returned, wrong flags, or ignored selectors.

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[gemini-code-assist]

Code Review

This pull request adds extensive and well-structured documentation for the new selector features, covering concepts, migration, syntax, and troubleshooting. The new documentation is a great addition. I've identified a few minor issues, such as a list numbering error and several instances of trailing whitespace that should be corrected for consistency. More significantly, there's a section on selector precedence in sync-configuration.md that appears to be confusing and potentially incorrect in its context, which I've flagged as a high-severity issue to ensure clarity.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud