Fix front matter for /onboard-with-notify

[emmagifford]

Description

We noticed /get-started/onboard-with-notify was rendering as an unstyled markdown page.

I looked at the file for that page and realised the usual 3 dashes weren't at the very top, which meant that the front matter wasn't recognised as front matter and the layout wasn't working.

I added the 3 dashes back in and it's loading fine again on the codespace preview, so this should fix the problem.

Context

This is needed to fix content that's not displaying with the correct styles and layout.

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.