How to: Deploy a custom plugin with Terraform in Konnect

## Jobs to be done (optional)
Deploying a custom plugin via Terraform is common, yet we don't document it.

## Definition of done
- Write a how to similar to the [on-prem one](https://developer.konghq.com/custom-plugins/get-started/deploy-plugins/#test-the-deployed-custom-plugin) we have that explains how to deploy a custom plugin with Terraform and Konnect
- Users will need to register the schema, like this section https://developer.konghq.com/custom-plugins/konnect-hybrid-mode/#add-a-custom-plugin-to-a-control-plane
- Then, they deploy the plugin with Terraform (https://github.com/Kong/terraform-provider-konnect/blob/main/examples/resources/gateway_custom_plugin_schema.tf)
- Can we also make this part of the "get started with custom plugins" series, so reusing steps [1-4 in this series](https://developer.konghq.com/custom-plugins/get-started/set-up-plugin-project/)?
- Look at the [Konnect custom plugin reference](https://developer.konghq.com/custom-plugins/konnect-hybrid-mode/) and determine if we can move some of the content to the how to. I think some of the content should stay as a reference. Make sure to link the how to and reference to each other.

## Information
Kapa ID: [b921bd26-dc94-4e1d-997c-d6bab67d283a](https://app.kapa.ai/93b55f65-cffb-4d0b-9da7-c8915a838dce/conversations/b921bd26-dc94-4e1d-997c-d6bab67d283a)

## Size
large

Start with the existing on-prem custom-plugin how-to, the Konnect custom plugin reference, and the terraform-provider-konnect example resource. Reuse the referenced get-started steps, document schema registration and Terraform deployment, then confirm the how-to and reference link to each other and that reference-only content remains appropriately placed.

Written by the indexing model from the issue text.