Phase banner include as HTML for OPG guidance

[jakecosgrove]

Description

This PR adds a phase banner that shows users the announcement for NHS digital-first strategy and links to /using-nhs-notify/nhs-digital-first-strategy.

Context

This banner acts as a CTA for new prospects who might come to the NHS Notify website following the announcement.

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]

HUGE thank you Jake for doing this PR and approving the other PR so quickly. We really appreciate your support!

Checked new content in the phase banner - looks great. Link to the new page works correctly.