CCM-9414 personalisation capital letter clarification and terminology changes

[emmagifford]

Description

As per the ticket, we've added inset text to clarify that personalisation fields must match PDS fields exactly, including letter cases.

We've also changed any references to 'placeholder' to 'personalisation field'.

Context

An integrator had problems with personalisation as their placeholders used incorrect capitalisation, so this new content aims to address that.

In a co-design session with Alex, we agreed to be consistent with our language and to stick with the term 'personalisation field' throughout the web UI and website. Sian mentioned that a previous service used this term and it didn't present any problems.

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.

[jakecosgrove]

Looks good to me!