docs: Cloud stub for shadow-linked cluster upgrade page (DOC-2217)

[Feediver1]

Companion to redpanda-data/docs#1819, which single-sources the new shadowing upgrade page.

Changes

• New stub modules/manage/pages/disaster-recovery/shadowing/upgrade.adoc that includes the single-source tag region of streaming:manage:disaster-recovery/shadowing/upgrade.adoc. In this component, env-cloud renders the upgrade-order rule plus maintenance-window sequencing guidance; the enterprise license note, rpk/rolling-upgrade instructions, and the bidirectional deferred-finalization procedure are self-managed-only and do not render here.
• Nav entry after Monitor Shadowing, matching the other shadowing stubs.

⚠️ Merge timing

Draft until the source page exists in the latest GA streaming version. The include resolves against the latest non-prerelease version of the streaming component, and the source page ships in 26.2 (currently beta) via redpanda-data/docs#1819 (itself stacked on #1818). Merging this before 26.2 GA would break the include. The same applies to the shared TIP added to setup.adoc in #1819, which xrefs this stub.

Review note

The Cloud-side guidance (sequence upgrades by scheduling the shadow cluster's maintenance window earlier in the Tuesday-first maintenance cycle than the source cluster's) is composed from the existing manage:maintenance.adoc facts — please have an SME confirm this is a supported way to order shadow-linked cluster upgrades in Cloud.

🤖 Generated with Claude Code

[netlify]

✅ Deploy Preview for rp-cloud ready!

Name	Link
🔨 Latest commit	6f36582
🔍 Latest deploy log	https://app.netlify.com/projects/rp-cloud/deploys/6a63d82ac2a23b0008576f51
😎 Deploy Preview	https://deploy-preview-648--rp-cloud.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[coderabbitai]

Important

Review skipped

Draft detected.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 73ffc8e9-7760-4cfe-ae8b-28e606a7f4cb

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch DOC-2217-shadowing-upgrade-cloud

---

_{Comment @coderabbitai help to get the list of available commands.}

[Feediver1]

Closing: upgrades for shadow-linked clusters will not be supported in Redpanda Cloud until August 28, so the upgrade page is intentionally not single-sourced for Cloud for now. The single-sourcing changes in redpanda-data/docs#1819 have been reverted. This can be revisited once Cloud support lands.