Added 'message cascade' concept into /routing-plans

[jakecosgrove]

Description

Content changes to explain the concept of primary and secondary message cascades. Message cascades were released as part of the 'parallel send' work that allows users to send recipients a message with multiple channels at the same time.

Context

This work is part of CCM-7032

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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Sensitive Information Declaration

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.