Update documentation for Getting Started with data migration

[jugal-chauhan]

Description

This change updates the "Getting started with data migration" documentation to improve clarity and accuracy. Specifically, it:

• Adds a new prerequisite about verifying the CDKToolkit Stack status.
• Updates the instructions for managing cluster passwords in AWS Secrets Manager.
• Clarifies the steps for setting up Bootstrap instance access.

Issues Resolved

Adds more clarification

Version

All

Frontend features

N/A

Checklist

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