Add TOTP authentication guide (#2196)

[Thavamani13]

Summary

• Adds a new totp-authentication.md guide under docs/sources/k6/next/examples/
• Covers real-world use cases requested in issue Create a guide for how to use the TOTP library #2196: 2FA login flows, payment step-up authentication, secret source integration, time bias/token expiry handling, multi-account parameterization, and code verification
• Links to existing totp jslib API reference and secret source docs throughout

Closes #2196

Test plan

• All code examples use valid k6 APIs (k6/http, k6/secrets, k6/data) and the totp jslib
• Frontmatter weight: 04 places the guide alongside other auth examples
• <!-- md-k6:skip --> annotation added to the bash CLI snippet to skip k6 execution in CI
• <K6_VERSION> placeholder used in all doc links (not hardcoded version strings)

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[codebien]

@inancgumus as you developed the feature, can you review this guide, please?

[inancgumus]

Thanks for picking this up! I'm reviewing this PR only from technicals. Some issues:

• totp.gen(30, 30) generates the previous window's code. So, you'd need gen(30, -30) for the next window.
• TOTP.verify() says "a code generated by the library is accepted by the server." But the TOTP library regenerates a code locally and compares strings. It doesn't talk to any server.
• The "Before you begin" section says clock drift of more than 30 seconds will cause failures. Whether that's true depends on the server (many accept adjacent windows).

@codebien I've technically reviewed this. Can you check the writing please? Thanks.

[codebien]

@Thavamani13 I'll continue the review as soon as you've addressed @inancgumus's request, so feel free to re-request a review as soon as you've pushed changes.