Clarify ARC quickstart authentication guidance

[KirtiRamchandani]

Why:

Closes: #37288

What's being changed (if available, include any code snippets, screenshots, or gifs):

The ARC quickstart now makes it clear that the PAT-based setup is a short path for completing the quickstart, not the recommended authentication approach for repository or organization runner scale sets. It points those users to GitHub App authentication and keeps the enterprise-level PAT classic requirement explicit.

Checks run:

• git diff --check
• npm run lint-content -- --paths content/actions/tutorials/use-actions-runner-controller/get-started.md

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
actions/tutorials/use-actions-runner-controller/get-started.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17 3.16	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17 3.16

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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