add GRPC-WEB to docs about gateway protocols

[mmatheson]

Please provide a description for what this PR is for.
GRPC-WEB is a valid port protocol, but not listed correctly in the docs.

And to help us figure out who should review this PR, please
put an X in all the areas that this PR affects.

• Configuration Infrastructure
• Docs
• Installation
• Networking
• Performance and Scalability
• Policies and Telemetry
• Security
• Test and Release
• User Experience
• Developer Infrastructure

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[istio-testing]

Hi @mmatheson. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[Arhell]

/ok-to-test

[ericvn]

This document is copied from the istio/api repo and the change needs to be made there.

See: https://github.com/istio/api/blob/master/networking/v1alpha3/gateway.proto#L599-L602

[istio-testing]

PR needs rebase.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[istio-testing]

@mmatheson: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
doc.test.profile_none_istio.io	133b556	link	true	/test doc.test.profile_none
doc.test.profile_demo_istio.io	133b556	link	true	/test doc.test.profile_demo
doc.test.multicluster_istio.io	133b556	link	true	/test doc.test.multicluster
doc.test.profile_default_istio.io	133b556	link	true	/test doc.test.profile_default
doc.test.profile_minimal_istio.io	133b556	link	true	/test doc.test.profile_minimal
doc.test.profile-demo_istio.io	133b556	link	true	/test doc.test.profile-demo
doc.test.profile-default_istio.io	133b556	link	true	/test doc.test.profile-default
doc.test.profile-none_istio.io	133b556	link	true	/test doc.test.profile-none
doc.test.profile-minimal_istio.io	133b556	link	true	/test doc.test.profile-minimal

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[craigbox]

I'm taking this forward for you in istio/api#2986.