[FLINK-38421] Blue/Green Deployments documentation

[schongloo]

What is the purpose of the change

This PR adds comprehensive documentation for the FlinkBlueGreenDeploymentController, explaining the design, control flow, and operational characteristics of Blue/Green deployments in the Flink Kubernetes Operator. The documentation provides both a conceptual overview and technical deep-dive to help users understand when and how to use Blue/Green deployments for zero-downtime Flink application upgrades.

Brief change log

• Added docs/content/docs/concepts/bluegreen-controller-flow.md documenting the FlinkBlueGreenDeploymentController
• Added Quick Start section for migrating from FlinkDeployment to FlinkBlueGreenDeployment
• Documented Blue/Green state machine with all 7 states
• Documented controller and state handler architecture
• Documented spec change detection and BlueGreenDiffType enum
• Documented ingress management and secondary resource tracking
• Documented health monitoring and error handling with abort grace period
• Added state machine and quick start configuration diagrams

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changes to the CustomResourceDescriptors: no
• Core observer or reconciler logic that is regularly executed: no

[gyfora]

cc @drossos