Add documentation about how to use KIC with cert-manager

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Does this enhancement require public documentation?

- [X] I have added an Acceptance Criteria item for adding and/or adjusting public documentation (if applicable)

### Problem Statement

Currently, we have an integration of cert-manager in the Helm chart, but it is not intended for proxying external traffic.

https://github.com/Kong/charts/tree/main/charts/kong#cert-manager-integration

We need a guide, similar to the "Get started" document so that users can easily integrate KIC with the cert-manager.

### Proposed Solution

_No response_

### Additional information

_No response_

### Acceptance Criteria

- [ ] Write an article about integrating and using KIC and cert-manager. Describe how to install and deploy them, and configure certificate access through Ingress or gateway API.

Start by reviewing the existing “Get started” documentation and the Kong chart’s cert-manager integration page. Define the article around installing and deploying KIC and cert-manager, then configuring certificate access through Ingress or the Gateway API; done means the guide covers each acceptance criterion clearly.

Written by the indexing model from the issue text.