Add 'Onboard with NHS Notify' page

[jakecosgrove]

Description

Migrates the 'Onboard with NHS Notify' page from the service catalogue to the main NHS Notify site.

This PR is to enable the onboarding team to review this page before promoting to the live site.

Some minor content tweaks have been made:

• an list of anchor links has been added to help users navigate to specific sections
• links included to relevant template pages in step 6

Context

This was requested by the onboarding team to ensure all information about NHS Notify was better contained in one place.

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.