update /pricing-and-commercial parent page

[emmagifford]

Description

On the /pricing-and-commercial parent page, I have:

• updated the letter pricing line with the new cost to send one double-sided letter using our standard postage
• added a line linking to the MOU and payment terms
• made the action link for text messages consistent with letters - updated 'More about' to 'Learn more about'

Context

A delivery manager in the commercial team flagged that the letter pricing line on the parent page wasn't up to date.

While I was editing that page, I noticed we hadn't added the link to the new-ish MOU and payment terms commercial agreements.

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.