DOC-1855 SR support for SASL/PLAIN in SM

[micheleRP]

Description

This pull request adds support for SASL/PLAIN authentication alongside existing methods for Self-Managed.

• Explicitly add SASL/PLAIN as a supported authentication method for the shadow link connection, updating instructions to mention SASL/SCRAM, SASL/PLAIN, TLS, and mTLS.
• Add configuration examples for SASL/PLAIN authentication (plain_configuration) in the client_options sections, alongside existing SASL/SCRAM examples, and ensure these are documented for non-cloud environments. [1] [2]
• Temporarily changed the referenced branch for cloud-docs in local-antora-playbook.yml from main to DOC-1621-Document-Cloud-Feature-Shadowing-Disaster-Recovery-Enterprise to show preview docs in Cloud don't contain this.

Resolves https://redpandadata.atlassian.net/browse/DOC-1855
Review deadline:

Page previews

Configure Shadowing: SM (search for PLAIN)
Configure Shadowing: Cloud (search for PLAIN)

Checks

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

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📝 Walkthrough

Walkthrough

This PR updates the local Antora playbook to source cloud documentation from a feature branch dedicated to documenting the shadowing disaster recovery feature. It also introduces SASL/PLAIN authentication configuration to the disaster recovery setup documentation, with conditional blocks ensuring this configuration only appears in non-cloud environments while preserving existing SASL/SCRAM, TLS, and mTLS options across multiple authentication sections.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

• local-antora-playbook.yml: Single branch reference update is straightforward
• setup.adoc: Repetitive pattern of identical SASL/PLAIN blocks added across multiple authentication sections with consistent conditional gating
• Primary focus should be verifying consistency of SASL/PLAIN additions across all authenticationConfiguration sections and confirming the ifndef/endif conditional syntax is correct in each location

Possibly related PRs

• DOC-1667 Document Shadow Link in Cloud #1491 — Modifies the same files with overlapping edits to both the playbook branch reference and setup.adoc authentication sections, addressing non-cloud SASL/PLAIN configuration and environment-specific single-sourcing.
• Fix local playbook #1492 — Updates the cloud-docs source branch in local-antora-playbook.yml with a different revision.
• DOC-1449 revert playbook #1164 — Modifies the same local-antora-playbook.yml cloud-docs source branch entry.

Suggested reviewers

• Feediver1
• paulohtb6

Pre-merge checks and finishing touches

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically summarizes the main change: adding SASL/PLAIN authentication support for Shadowing/Replication in Self-Managed.
Linked Issues check	✅ Passed	The PR changes align with both linked issues: DOC-1855 (adds SASL/PLAIN support for SM) and DOC-1621 (coordinates Cloud documentation via branch reference change).
Out of Scope Changes check	✅ Passed	All changes are in scope: SASL/PLAIN authentication additions to setup.adoc and temporary cloud-docs branch reference change are directly aligned with DOC-1855 and DOC-1621 objectives.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description check	✅ Passed	The pull request description is well-structured and comprehensive, following the template with all required sections filled out: a clear description of changes, JIRA reference (DOC-1855), page previews with direct links, and relevant checkboxes marked.

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[rpdevmp]

LGTM