CCM-15887 update message plans

[emmagifford]

Description

I've updated:

• /using-nhs-notify/message-plans
• /using-nhs-notify/upload-a-letter#page-breaks

Context

Now that the message plans feature is almost live in the logged in part of Notify, we need to update the old 'routing plans' content to bring it in line with the transactional content, particularly the more user-friendly terminology of 'message plans'.

I've tried to go back to basics in explaining the main things message plans allow you to do - choose message channels, assign templates and choose fallbacks. I removed the terminology of 'cascades' because that's not a concept we use in the transactional service. Also, 'secondary cascades' was describing 'parallel send', where a message is sent to multiple channels at the same time, and this isn't a current feature of web UI.

The draft content is in Confluence, which has been approved by our product owner.

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

To ensure the utmost confidentiality and protect your and others privacy, we kindly ask you to NOT including PII (Personal Identifiable Information) / PID (Personal Identifiable Data) or any other sensitive data in this PR (Pull Request) and the codebase changes. We will remove any PR that do contain any sensitive information. We really appreciate your cooperation in this matter.

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

[emmagifford]

Thanks to James's eagle eyes, I've made some further updates:

• changed all instances of 'routing plans' to 'message plans' across other pages, including the landing page. I've not changed 'routing plan ID' as this is in the backend and would need a future update from devs
• changed the sentence about users setting up their own message plans to future tense as this feature isn't live yet