(DOCS-3777) Update Pivotal PKS setup

[buraizu]

What does this PR do?

Add information to the Pivotal PKS documentation setup section, which was removed from the main Pivotal Platform documentation in the following documentation repo PR: https://github.com/DataDog/documentation/pull/14955

Motivation

DOCS-3777

Additional Notes

Preview link: https://docs-staging.datadoghq.com/docs3777/update-pull-config-preview/integrations/pivotal_pks/#setup

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have changelog/ and integration/ labels attached