Kubernetes/ Envoy AI-Gateway doc needs change

[srampal]

Describe the bug
In deploy/kubernetes/README.md as well as in deploy/kubernetes/ai-gateway/README.md the instruction is
to run kubectl apply -k deploy/kubernetes/. However this means we always run all manifests in the kubernetes folder and below sub-folders which is not what we want. At the deploy/kubernetes/ level we are only deploying semantic-router but not ai-gateway since the same yaml files are also for other gateways like istio or others which will be sub-folders. But that will end up deploying all the different gateways because all manifests in all sub-folders gets applied. Hence this documentation needs to be fixed.

To Reproduce
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1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
Updated documentation structure to prevent overlap of manifests for different use cases.

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Additional context
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[Xunzhuo]

yep, we can customize it via better UX when supporting helm chart, and dynamic API.