feat: Add client credentials guide for NGINXaaS Google

[nparikh03]

Proposed changes

Add new how-to guide for setting up OAuth2 client credentials for programmatic access to NGINXaaS for Google APIs.

This guide covers:

Creating and managing client credentials from the NGINXaaS console
Retrieving and displaying client information
Deleting expired credentials
Exchanging client credentials for access tokens
Best practices for credential security and expiration
This enables machine users such as Terraform, CI/CD pipelines, and automation tools to authenticate securely with the NGINXaaS API.

Checklist

Before sharing this pull request, I completed the following checklist:

• I read the Contributing guidelines
• My branch adheres to the Git conventions
• My content changes adhere to the F5 Technical Writing Style Guide
• If my changes involve potentially sensitive information¹, I have assessed the possible impact
• I have waited to ensure my changes pass tests, and addressed any discovered issues

Footnotes

1. Potentially sensitive information includes personally identify information (PII), authentication credentials, and live URLs. Refer to the style guide for guidance about placeholder content. ↩

[github-actions]

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[nparikh03]

I have hereby read the F5 CLA and agree to its terms

[github-actions]

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[Copilot]

Pull request overview

This PR adds a new NGINXaaS for Google Cloud quickstart how-to guide that explains how to create and use OAuth2 client credentials for programmatic access to the NGINXaaS API (for example, automation via Terraform or CI/CD).

Changes:

• Adds a new “client credentials” guide with console-based creation/retrieval/deletion steps.
• Documents token exchange and authenticated API call examples (cURL), plus best practices and error examples.
• Adds a quick reference table summarizing limits and token/credential properties.

[puneetsarna]

@nparikh03 : Can you please make sure to address this https://github.com/nginx/documentation/pull/2052/changes#r3431835287. I will review the rest of it for now.

[JTorreG]

approved with 2 nits