docs: update 1-8-go-live-readiness.md

[habib424]

Drafted by Ship Ship 🚢 — confidence 🟢 high

What changed

Added a "Good to know" callout in the Data migration requirements section noting that migrated contracts, payable/receivable invoices, and journal entries display a read-only "Migration cutoff date" field on their detail page, giving customers a way to verify the cutoff date used for a migrated record.

Source PRs

• axolotl#4658 — feat: display migration cutoff date

How to use this PR

1. Review the diff against the existing article — focus on accuracy and customer voice.
2. Merge if it reads well. Edit and push to this branch if it needs adjustment.
3. Close without merging if the article doesn't actually need this update.

Once we have a track record of clean drafts, we'll graduate to auto-merge with a delay buffer. For now: human-in-the-loop.