DOC-1939 add schema validation topic in cloud docs

[micheleRP]

Description

Single-sourcing for this in docs repo done in redpanda-data/docs#1565

This pull request introduces documentation for schema ID validation in Redpanda Cloud. It adds the single-sourced page for schema ID validation, updates navigation to include this topic, and announces the feature in the "What's New" section.

• Added a new page schema-id-validation.adoc that explains server-side schema ID validation, its configuration, and its impact on Redpanda brokers.
• Updated navigation (nav.adoc) to include a link to the new schema ID validation documentation.
• Announced the schema ID validation feature in the "What's New" section, including a summary of how to enable it and its potential impact on CPU load.

Resolves https://redpandadata.atlassian.net/browse/DOC-1939
Review deadline:

Page previews

Checks

• New feature
• Content gap
• Support Follow-up
• Small fix (typos, links, copyedits, etc)

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📝 Walkthrough

Walkthrough

This pull request updates documentation to introduce schema ID validation support in Redpanda Cloud. Changes include updating the Antora content source configuration to fetch from a feature branch, adding a new navigation entry for schema ID validation documentation, documenting the feature in the "What's New" section for February 2026, and creating a new documentation page explaining server-side schema ID validation for clients using SerDes.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related PRs

• Fix local playbook #463: Modifies the same local-antora-playbook.yml file to update content-source branch configuration.

Suggested reviewers

• Feediver1
• mattschumpert

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	⚠️ Warning	All changes are directly related to documenting schema ID validation as specified in DOC-1939; however, the playbook branch change appears unrelated to the documentation updates.	Clarify whether the local-antora-playbook.yml branch change from main to DOC-1943-single-source-schema-id-validation is intentional and necessary for this PR, or if it should be reverted.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: adding documentation for schema validation in cloud docs, directly matching the primary objective of the PR.
Linked Issues check	✅ Passed	All coding requirements from DOC-1939 are met: schema ID validation documentation created, cluster property enable_schema_id_validation documented, and operational implications (CPU impact) noted.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description check	✅ Passed	PR description provides clear summary of changes, resolves issue DOC-1939, includes feature checkbox, but lacks page preview links.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch DOC-1939-Add-Schema-Validation-topic-in-general-and-cluster-property-enable_schema_id_validation-to-cloud-docs

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[Feediver1]

lgtm