docs(copilot-cli): quickstart and auth method comparison for authenticate-copilot-cli

[MAXDVVV]

Summary

Addresses part of #44543 (help wanted, copilot label):

• Quickstart — minimal steps to install, authenticate, and verify Copilot CLI
• Choose an authentication method — comparison table (GitHub.com login vs PAT vs GHEC/GHES)
• Troubleshooting link to existing auth troubleshooting article
• CI tip — note on using fine-grained PATs or OIDC instead of long-lived tokens in CI

Test plan

• Markdown renders in local preview
• Links point to existing docs paths
• Docs team review for style/terminology alignment

Fixes #44543 (partial)

[github-actions]

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Table of review links

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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
copilot/how-tos/copilot-cli/set-up-copilot-cli/authenticate-copilot-cli.md	fpt ghec	fpt ghec

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

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