Revisit manage aigw with Kongctl doc

## Definition of done
- Revisit `app/_how-tos/konnect-platform/manage-ai-gateway-with-kongctl.md` and republish it
- add links to the kongctl docs where it was removed.

## Information
I removed this from the 1.14 kongctl in order to get everything published. This doc needs to be reviewed and edited to conform to our how-to standards.

Here are some of my ideas:
- Review this from top to bottm
- Add steps for any friction encountered like using `echo` to make the `ai-gateway.yaml` file.
- remove `skip_product: false`
- potentially use the quickstart script if it makes sense

## Size
M

Start with app/_how-tos/konnect-platform/manage-ai-gateway-with-kongctl.md and review it from top to bottom against the project's how-to standards. Restore links to the kongctl docs where needed, clarify steps such as creating ai-gateway.yaml with echo, remove skip_product: false, and assess whether the quickstart script fits. Done means the reviewed document is republished and its steps are complete and clear.

Written by the indexing model from the issue text.