CCM-13489: add standard letter docx upload page

[harrim91]

Description

Companion to NHSDigital/nhs-notify-web-gateway#125

Adds 4 new pages

• /upload-standard-english-letter-template
• /upload-large-print-letter-template
• /upload-other-language-letter-template
• /upload-british-sign-language-letter-template

Also:

• Updates / removes some test ids. If passing test ids through markdown, this can be done in overrides, rather than via props passed to ContentRenderer or MarkdownContent. It was causing lots of warnings in tests etc.
• Updates some tests to target things via accessible role rather than test id.
• BackLink component exports an alternate version which uses Next.js Link and doesn't require it being wrapped and using the deprecated legacyBehaviour / passHref props.
• Adds a docx test fixture and renamed the directory from pdf-upload to letters , so lots of the changes are just renaming (and removing BOM header from some CSV's)

Context

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
• If I have used the 'skip-trivy-package' label I have done so responsibly and in the knowledge that this is being fixed as part of a separate ticket/PR.

---

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.