Recommend GitHub App authentication in the ARC quickstart

[sushant-me]

Why

The ARC quickstart currently instructs readers to generate a legacy personal access token (classic) as the default way to authenticate ARC to GitHub. For repository and organization runners, GitHub recommends authenticating with a GitHub App instead; a personal access token (classic) is only required for runners at the enterprise level.

What changed

• The authentication step in the quickstart now points readers to the existing authentication guidance and recommends a GitHub App for repository and organization runners, while noting that a personal access token (classic) is required for enterprise-level runners.
• Added a note near the PAT-based example clarifying that readers using a GitHub App should configure \githubConfigSecret\ with the app credentials (\github_app_id, \github_app_installation_id, and \github_app_private_key) instead of \github_token.

Verification

• Anchor references in the updated links match existing section headings in the authentication guide.
• No content was removed from the PAT-based example, which remains valid (and required) for enterprise-level configurations.

Closes #37288

[github-actions]

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Table of review links

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Source	Review	Production	What Changed
actions/tutorials/use-actions-runner-controller/get-started.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17

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

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[subatoi]

Reopening—closed in error, apologies.