Website: Clean up “Non-preempting PriorityClasses” page section

[sftim]

This is a cleanup issue

This issue is about clean-up work for https://kubernetes.io/docs/concepts/configuration/pod-priority-preemption/#non-preempting-priority-class

1. The table of contents does not list “Non-preempting PriorityClasses”.
2. The section for Non-preempting PriorityClasses (alpha) should use a feature state shortcode.

Proposed Solution:

1. Change the heading level for:

markdown ### Non-preempting PriorityClasses (alpha) {#non-preempting-priority-class}
so that this section appears in the table of contents.

1. ~Remove~ revise the text

1.15 adds the `PreemptionPolicy` field as an alpha feature.
It is disabled by default in 1.15,
and requires the `NonPreemptingPriority`[feature gate](/docs/reference/command-line-tools-reference/feature-gates/
) to be enabled.

and add a feature state shortcode:
```markdown
{{< feature-state for_k8s_version="1.15" state="alpha" >}}
```

Page to Update:
https://kubernetes.io/docs/concepts/configuration/pod-priority-preemption/

Additional Information:
/kind cleanup
/priority backlog
/good-first-issue

[k8s-ci-robot]

@sftim:
This request has been marked as suitable for new contributors.

Please ensure the request meets the requirements listed here.

If this request no longer meets these requirements, the label can be removed
by commenting with the /remove-good-first-issue command.

In response to this:

This is a cleanup issue

This issue is about clean-up work for https://kubernetes.io/docs/concepts/configuration/pod-priority-preemption/#non-preempting-priority-class

1. The table of contents does not list “Non-preempting PriorityClasses”.
2. The section for Non-preempting PriorityClasses (alpha) should use a feature state shortcode.

Proposed Solution:

1. Change the heading level for:

markdown ### Non-preempting PriorityClasses (alpha) {#non-preempting-priority-class}
so that this section appears in the table of contents.

1. Remove the text

1.15 adds the `PreemptionPolicy` field as an alpha feature.
It is disabled by default in 1.15,
and requires the `NonPreemptingPriority`[feature gate](/docs/reference/command-line-tools-reference/feature-gates/
) to be enabled.

and add a feature state shortcode:
markdown {{< feature-state for_k8s_version="1.15" state="alpha" >}}

Page to Update:
https://kubernetes.io/docs/concepts/configuration/pod-priority-preemption/

Additional Information:
/kind cleanup
/priority backlog
/good-first-issue

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[sftim]

If you are new to contributing to the website, want to help with this issue and have questions, reply here and someone from SIG Docs will offer you advice.

(It's also OK to ask questions if you're already participating; it'd be great to leave this issue for a new contributor though).

[dethi]

/assign

[sftim]

PS. It's important that readers get to learn about the feature gates named PreemptionPolicy and NonPreemptingPriority.