Add renamed legacy migrator components

[JakeSCahill]

Description

In redpanda-data/rp-connect-docs#317, we add the new legacy_ versions of the Redpanda migrator components. This PR adds those components to Redpanda Cloud docs and adds the migration guide for moving to the new unified Redpanda migrator components.

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 for Redpanda Connect's migrator components. The changes introduce "legacy" prefixed documentation for the older redpanda_migrator_offsets component while removing the previous documentation, add a new migration guide for transitioning to the unified Redpanda Migrator, update the navigation tree accordingly, and modify the Antora playbook source branch from main to auto-docs/update-rpcn-connector-docs. All changes are documentation-focused with no code modifications.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

The changes consist of homogeneous documentation updates following a repetitive pattern: adding legacy-prefixed AsciiDoc scaffolding files, removing obsolete documentation pages, updating navigation references, and modifying a configuration file. While affecting multiple files, the changes are straightforward and low-complexity—primarily boilerplate documentation structure with include directives pointing to external sources.

Suggested reviewers

• micheleRP
• Feediver1
• kbatuigas

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title Check	✅ Passed	The PR title "Add renamed legacy migrator components" directly corresponds to the main changes in the pull request. The changeset focuses on adding documentation for new legacy-prefixed versions of the Redpanda migrator components (legacy_redpanda_migrator and legacy_redpanda_migrator_offsets) across both inputs and outputs, along with a migration guide. The title is concise, clear, and specific enough that a teammate scanning history would understand the primary purpose of the change. It avoids vague language or unnecessary noise.
Description Check	✅ Passed	The PR description adequately explains the changes, referencing the related upstream change in rp-connect-docs/pull/317 and clearly stating that this PR adds the new legacy-prefixed migrator components to the Redpanda Cloud documentation plus a migration guide. The description follows the general structure of the template with the Description section populated and a Checks section provided. However, some template elements are incomplete or missing: no specific issue number is referenced, no review deadline is set, and page preview links are not filled in (though the placeholder structure is present). Since the core content and intent of the changes are clearly communicated despite these missing non-critical details, the description meets the threshold of being mostly complete.

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[paulohtb6]

Does the migrator page has info about the breaking changes? if yes, we should make that info explicit here