docs: improve cross-links for scheduler preemption

[tgross]

Fix a broken link from the preemption concepts docs to the relevant API. Also include a link to the relevant command.

Ref: #25038
Preview link: https://nomad-m6t6tq8jc-hashicorp.vercel.app/nomad/docs/concepts/scheduling/preemption#details

[aimeeu]

Thanks Tim!

[EugenKon]

Hi. I am not so experienced, thus I'll highlight parts that are not so clear for me:

Preemption is enabled by default for system jobs. Operators can use the scheduler config API endpoint to disable preemption.

The link above has nothing about scheduler. We need to click two pages deeper to find interested information. It would be better to put direct link to the information for scheduler config: https://developer.hashicorp.com/nomad/docs/commands/operator/scheduler/set-config

https://developer.hashicorp.com/nomad/docs/commands/operator/scheduler/set-config#preempt-system-scheduler

Please note, that this part does not say that this scheduler is enabled by default. But this part https://developer.hashicorp.com/nomad/docs/concepts/scheduling/preemption#details mentions that: Preemption is enabled by default for system jobs. (I am not sure about that it is enabled by default. If that was true, then I had not met situation when Nomad client is without system jobs: #25061)

As you commented here It looks like system jobs do not have enabled by default preemption.

https://developer.hashicorp.com/nomad/docs/concepts/scheduling/preemption#details

It would be worth to extend this example to show that if preemption for service jobs was not enabled it will not preempt other jobs.

Thanks.

[tgross]

The link above has nothing about scheduler. We need to click two pages deeper to find interested information. It would be better to put direct link to the information for scheduler config: https://developer.hashicorp.com/nomad/docs/commands/operator/scheduler/set-config

Please note, that this part does not say that this scheduler is enabled by default. But this part https://developer.hashicorp.com/nomad/docs/concepts/scheduling/preemption#details mentions that: Preemption is enabled by default for system jobs.

That's literally what this PR does, mate. Look at the diff or the preview:

[EugenKon]

@tgross I checked the diff, but that format was not so clear. preview link helped. Thanks.

But this still does not documents that this option is enabled by default: https://developer.hashicorp.com/nomad/docs/commands/operator/scheduler/set-config#preempt-system-scheduler

3. Example also was not updated.

[EugenKon]

@tgross Broken link about schedulers: https://developer.hashicorp.com/nomad/docs/configuration/server#default_scheduler_config

Also it would be nice to have a full list of possible values that could be configured here: https://developer.hashicorp.com/nomad/docs/configuration/server#enabled_schedulers

[tgross]

@EugenKon PRs welcome

[EugenKon]

So all is actually is not default value:

https://developer.hashicorp.com/nomad/docs/configuration/server#enabled_schedulers