Add Documentation for the Queue Proxy Option feature (extensions to QP)

[davidhadas]

Document how to add and configure a runtime extension to QP Documentation should include:

• What is a QP Extension
• How does one add a QP Extension to the QP Image
• How does one configure the cluster to enable the use of QP Extensions
• How does one activate and configure a QP Extension on a specific service
• Examplify adding, enabling, activating, and configuring a "Test-Gate" from knative.dev/security-guard with a QP Extension Additional context Code added in https://github.com/knative/serving/pull/13162

[davidhadas]

/assign davidhadas