Add small section about what happens to Gists if you change username.

[billythekid]

Why:

Closes #20769

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

Adding a small section about Gists and how they're affected if you change username.

Check off the following:

• I have reviewed my changes in staging (look for the "Automatically generated comment" and click the links in the "Preview" column to view your latest changes).
• For content changes, I have completed the self-review checklist.

Writer impact (This section is for GitHub staff members only):

• This pull request impacts the contribution experience
  • I have added the 'writer impact' label
  • I have added a description and/or a video demo of the changes below (e.g. a "before and after video")

[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]

Automatically generated comment ℹ️

This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
account-and-profile/setting-up-and-managing-your-personal-account-on-github/managing-personal-account-settings/changing-your-github-username.md	fpt ghec ghes@ 3.6 3.5 3.4 3.3 3.2	fpt ghec ghes@ 3.6 3.5 3.4 3.3 3.2

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server
ghae: GitHub AE

[cmwilson21]

👋 @billythekid Thank you for opening a PR and linking it to your issue! ✨

I'll get this triaged for review! ⚡

[PIYUSH-MISHRA-00]

An explanatory section about what to Gists if you change username.

[4bambi]

I suppose I'm learning

[vgrl]

Thanks @billythekid ⚡ , I'm going to make a tiny change and then I'll approve and merge this.

[vgrl]

Update Gists to gists

[vgrl]

Looks good on preview 🎉

[github-actions]

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