chore: update authz docs to clarify dif between v1

[markphelps]

Re: https://github.com/orgs/flipt-io/discussions/4583

[Copilot]

Pull Request Overview

This PR updates the Flipt v2 authorization documentation to clarify the differences between v1 and v2 authorization systems. The documentation enhancement addresses community discussion about distinguishing these versions.

• Adds comprehensive explanation of v2's environment-aware authorization model
• Provides complete example policies and data structures for v2 implementation
• Documents migration path from v1 to v2 with specific breaking changes

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