Modify documentation for Github Pages custom domain

[sakaiist]

Why:

Beginners rotely following the instructions may miss that their DNS provider does not accept the full URL (e.g., www.example.com) in the 'Host' field of the CNAME record. For example, namecheap only accepts the third-level domain (e.g., www). As a beginner, this can be an annoying detail to miss given the expectation of DNS propagation delay.

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

Added a short aside that a diligent beginner might catch on reading through tutorial.

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 review environment.

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[github-actions]

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	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

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

🤖 This comment is automatically generated.

[Sharra-writes]

Thanks for opening this PR! I'll get it triaged for review.

[github-actions]

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