Expand documentation for adding an custom Job type to Kueue via external controller

[dgrove-oss]

What would you like to be added:

Rework the page on adding a custom job type to Kueue to equally describe adding an external extension.

Why is this needed:

Kueue actually supports external extension fairly well and documenting it would help grow the community.

Completion requirements:

This enhancement requires the following artifacts:

• Design doc
• API change
• [ x ] Docs update

The artifacts should be linked in subsequent comments.

[dgrove-oss]

I'm planning to do this based on our experience with building the AppWrapper controller. The main question I had was whether you would prefer the final doc to be one page with 2 sections or a short summary page with two child pages each giving the details of internal and external.

[tenzen-y]

I'm planning to do this based on our experience with building the AppWrapper controller. The main question I had was whether you would prefer the final doc to be one page with 2 sections or a short summary page with two child pages each giving the details of internal and external.

@dgrove-oss IMO, we depend on the amount of differences between the existing integration approach (https://kueue.sigs.k8s.io/docs/tasks/dev/integrate_a_custom_job/) and new external integration.

@alculquicondor @mimowo Any thought?

[alculquicondor]

The existing page looks small enough, so maybe it would be fine to add it there. But don't stress about it and open a new page if it feels right.