docs: document service account token daily rate limit

[micheleRP]

Summary

• Adds a Service account token rate limits subsection under Authenticate to the Cloud API in modules/security/pages/cloud-authentication.adoc, covering the HTTP 429 response, token caching guidance, and the path to contact Redpanda Support for higher daily limits.
• Adds a matching May 2026 entry to modules/get-started/pages/whats-new-cloud.adoc linking back to the new subsection.

Reference (internal): cupboard engineering/auth/2026-05-22-service-account-token-rate-limit.md and the matching runbook.

Preview pages

• Authentication (updated)
• What's New in Redpanda Cloud (updated)

Test plan

• Local Antora build completes without new warnings or broken xrefs for the changed files
• Security > Authentication > Service account token rate limits renders below Authenticate to the Cloud API and above Enable mTLS authentication
• Get Started > What's New > May 2026 > Service account token rate limits renders and the See link resolves to the new anchor on the authentication page
• No em dashes; sentence-case headings; second-person voice; passes docs-team-standards review on the touched files

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📝 Walkthrough

Walkthrough

This PR documents a new service account token rate-limiting feature in Redpanda Cloud. It adds two complementary documentation sections: one entry in the May 2026 "what's new" changelog, and one detailed section in the cloud authentication guide. Together, they explain the organization-level daily quota for service account access token requests, describe the HTTP 429 response when the limit is exceeded, recommend token caching strategies, and provide an escalation path for organizations requiring higher limits.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Possibly related PRs

• redpanda-data/cloud-docs#555: Both PRs touch the same "service account" documentation around authorization/authentication behavior—this PR adds token rate-limit/HTTP 429 quota guidance for service account access tokens, while the retrieved PR updates RBAC UI docs for assigning scoped roles to service accounts.

Suggested reviewers

• Mateoc
• kbatuigas
• Feediver1

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: documenting service account token daily rate limits across two documentation files.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description is well-structured with clear summary, preview pages, and test plan, but diverges from the template by missing the issue reference and checkbox verification.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs-service-account-token-rate-limits

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[Mateoc]

LGTM
I think we should include how Auth0-Organization-Quota-Limit header works, is the way to track the quota. Here is some info on how it works https://github.com/redpanda-data/cupboard/blob/main/engineering/auth/2026-05-22-service-account-token-rate-limit.md ping me for clarification