docs(service users): improve client id and client secret docs

[mffap]

Which Problems Are Solved

• Misaligned heading
• Curl with auth header is hard to understand without the link

How the Problems Are Solved

Instead of explaining how to create a proper encoding, just use --user flag for client_id and client_secret

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Name	Status	Preview	Comments	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 29, 2024 9:13am

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[github-actions]

🎉 This PR is included in version 2.54.0 🎉

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