GitHub Pages: Adding note to remove default DNS records

[internetisaiah]

Why:

Closes: #36158

What's being changed (if available, include any code snippets, screenshots, or gifs):

I set up GitHub pages for the first time and ran into issues because my DNS provider (Squarespace) automatically adds an A record and apex domain for newly created domains. After doing some research it appears this is common for many DNS providers (i.e. Wix, GoDaddy, etc.) so I figured it was worth notating to help other readers.

Feel free to move it to a different part of the page, or even incorporate it into an existing paragraph instead of my note alert--whatever you think is best!

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the preview environment.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[github-actions]

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Content directory changes

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Source	Preview	Production	What Changed
pages/configuring-a-custom-domain-for-your-github-pages-site/managing-a-custom-domain-for-your-github-pages-site.md	fpt ghec	fpt ghec

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Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server
This table is posted from the Content Changes Table Comment workflow.

[github-actions]

Thanks for opening a pull request! We've triaged this issue for technical review by a subject matter expert 👀

[isaacmbrown]

Thanks for the contribution @internetisaiah! I got this confirmed by an SME and made a small change to match the docs tone slightly more. I'll get this merged for you now 🚀

[github-actions]

Thanks very much for contributing! Your pull request has been merged 🎉 You should see your changes appear on the site in approximately 24 hours. If you're looking for your next contribution, check out our help wanted issues ⚡

[internetisaiah]

@isaacmbrown , awesome! thanks for getting this merged so quickly. have a great rest of your day 🫡