Add Workflows Authentication page

[urseberry]

What does this PR do?

Adds a page to the Workflows product documentation about Authentication. Originally, this section was called Setup, but I realized that it only has to do with authentication, so I renamed it.

Motivation

This section corresponds to the following outline, agreed upon by the Documentation team and Sarjeel:

Setup/Prerequisites

1. Explain that workflows use integration configurations
2. Manage Workflow Integrations via Workflow Connections a. (P1) Create New Connection b. (P1) Restrict Workflow Connection Usage c. (P2) Edit Workflow Connection d. (P3) Delete Workflow Connection

Additional Notes

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Reviewer checklist

• [ ] Review the changed files.
• [ ] Review the URLs listed in the Preview section.
• [ ] Check images for PII
• [ ] Review any mentions of "Contact Datadog support" for internal support documentation.

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/uchen/workflow-setup/workflows/setup

[urseberry]