Add about gitignore global default config at ignoring-files.md (#21863)

[aokabin]

Why:

Closes: #21863

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Source	Preview	Production	What Changed
get-started/getting-started-with-git/ignoring-files.md	fpt ghec ghes@ 3.10 3.9 3.8 3.7 3.6 ghae	fpt ghec ghes@ 3.10 3.9 3.8 3.7 3.6 ghae

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[cmwilson21]

@aokabin Thanks so much for submitting a PR! We appreciate the screenshots, too.

I'll get this triaged for review ⚡

And welcome to the community! If you are looking for more ways to contribute, please take a look at our help wanted section to find open issues you can work on.

[jhosman]

Thanks for opening a PR with this fix @aokabin! ❤️

I’ve made a few additional commits to expand the first paragraph to include an example, and to remove all mentions of the ~/.gitignore_global file. (Since ~/.config/git/ignore has been the Git default for quite a while now, it should be safe to only recommend this single method going forward.)

While I was in there, I also updated the formatting of all filenames in the article to use backticks instead of italic format, per the style guide.

We'll go ahead and get this merged for you once tests are passing! Thanks so much! 🚀 ✨

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