Update K8s API gateway guide for new ngrok-operator

[joelhans]

Making a few QOL improvements and aligning with new product updates in ngrok/ngrok-operator:

1. Made the example API create its own namespace, removing the kubectl create namespace prod step
2. Updated the helm command to use the new ngrok/ngrok-operator chart! 🥳
3. Moved the operator to the ngrok-operator namespace now that they can work in different namespaces.
4. Moved to on_http_request instead of inbound now that phase-based naming works in the operator.
5. Fixed the curl loop example to use POST, which will better show the rate limiting in action.

[vercel]

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Name	Status	Preview	Updated (UTC)
ngrok-docs	✅ Ready (Inspect)	Visit Preview	Nov 15, 2024 1:39am

[DevMandy]

I retested the whole thing and it's golden!