Update custom-domain.mdx

[dino475]

Documentation changes

adding an og:description to the front matter of the custom domains page to test this ticket: https://linear.app/mintlify/issue/CUS-978/description-front-matter-isnt-appearing-in-google-previews

Closes

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For Reviewers

When reviewing documentation PRs, please consider:

✅ Technical accuracy

• Code examples work as written
• Commands and configurations are correct
• Links resolve to the right destinations
• Prerequisites and requirements are accurate

✅ Clarity and completeness

• Instructions are clear and easy to follow
• Steps are in logical order
• Nothing important is missing
• Examples help illustrate the concepts

✅ User experience

• A new user could follow these docs successfully
• Common gotchas or edge cases are addressed
• Error messages or troubleshooting guidance is helpful

[dino475]

@ethanpalm if you get a second to help me merge this

[ethanpalm]

lgtm