Add font embedding content and fix heading typo

[emmagifford]

Description

I have:

• added a new section on using fonts other than Noto Sans and how to embed them correctly
• fixed a typo on the /message-plans page where I'd not added ## markdown on a heading

Context

Certain non-NHSE users aren't allowed to use NHS branding. The developers identified problems in the Carbone software where files with fonts that weren't embedded weren't uploading. We need to add content for the workaround while clarifying that you can only use different fonts in exceptional circumstances.

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.