DOC-1812: Add single-sourced Schema Registry Contexts page

[micheleRP]

Summary

• Single-sources the Schema Registry Contexts page from the docs repo (Doc-1996: Schema Registry Contexts docs#1658)
• Creates a stub page at modules/manage/pages/schema-reg/schema-reg-contexts.adoc with an include directive
• Adds nav entry under Schema Registry
• Adds What's New April 2026 entry

fixes https://redpandadata.atlassian.net/browse/DOC-1812

Preview pages

Schema Registry Contexts
What's New

Dependencies

• Depends on: Doc-1996: Schema Registry Contexts docs#1658 (must merge first so the include resolves on main)
• I added // tag::single-source[] / // end::single-source[] tags to the docs PR so the include works

Before merge

• Revert local-antora-playbook.yml: Replace Feediver1-patch-7 back to main in the docs repo branches list (line 18). This override exists only for Netlify preview.
• Ensure docs PR Doc-1996: Schema Registry Contexts docs#1658 is merged first

Test plan

• Verify Netlify preview renders the Schema Registry Contexts page with content (not empty)
• Verify nav entry appears in the Schema Registry section
• Verify What's New entry renders with working link

🤖 Generated with Claude Code

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[coderabbitai]

📝 Walkthrough

Walkthrough

This pull request updates documentation to introduce Schema Registry Contexts, a new Redpanda feature. The changes include: updating the Antora build configuration to pull from a feature branch (Feediver1-patch-7), adding a navigation entry for the new Schema Registry Contexts documentation page, documenting the feature in the April 2026 "What's New" section, and adding a dedicated documentation page explaining that contexts create isolated namespaces for schemas, subjects, and configuration with independent schema ID counters, mode settings, and compatibility settings.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~4 minutes

Possibly related PRs

• DOC-1586 single source Manage Throughput in cloud docs #388: Modifies the same local-antora-playbook.yml to point to a feature branch and adds new navigation entries and documentation pages under modules/manage, following an identical pattern.
• Fix local playbook #463: Updates the same Antora content-source branch list in local-antora-playbook.yml to include feature branches.
• Add migration pages to Cloud #511: Modifies the same local-antora-playbook.yml branch configuration and adds navigation entries in modules/ROOT/nav.adoc.

Suggested reviewers

• Feediver1
• mattschumpert
• sago2k8

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: adding a single-sourced Schema Registry Contexts documentation page, directly matching the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description check	✅ Passed	The PR description is mostly complete with summary, preview pages, dependencies, and test plan, but lacks required template sections like issue link and explicit feature/fix classification.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch DOC-1812-Document-Cloud-Feature-Schema-Registry-Contexts-Serverless-etc

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