RDSC-4454 Document RDI maintenance deletion limit

[ZdravkoDonev-redis]

Summary: Add a note to the RDI Delete pipeline docs that data pipelines cannot be deleted while the target Redis Cloud cluster is in maintenance mode, and tell users to retry deletion after maintenance is complete. Validation: git diff --check.

[github-actions]

Staging links:
https://redis.io/docs/staging/codex-rdsc-4454-rdi-maintenance-docs/operate/rc/databases/rdi/view-edit/

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

---

^{Security scan by Jit}

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}