Documentation and examples needed for HTTP Request Mirroring

[robscott]

What would you like to be added:
Although we have conformance test coverage for request mirroring, we don't have any corresponding documentation. This would fit well as a guide, similar to the existing redirect one.

Why this is needed:
This is an important feature of the API and it's currently completely missing from our documentation.

[robscott]

/help

[k8s-ci-robot]

@robscott:
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[sawsa307]

/assign

I would like to work on this one.