Add generation of custom-named SSH Keys for 'Authentication' section

[Allyson04]

Why:

Closes #19166

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

Advising user to create a SSH key with custom name instead of default name, as this last one can conflict with another previously created SSH keys, although using the default name and location is still a possibility. Changing this for Mac, Linux and Windows, as the location for each of them is different.

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  • 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")

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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

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Source	Preview	Production	What Changed
authentication/connecting-to-github-with-ssh/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent.md	fpt ghec ghes@ 3.6 3.5 3.4 3.3 3.2 ghae	fpt ghec ghes@ 3.6 3.5 3.4 3.3 3.2 ghae

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

[cmwilson21]

@Allyson04 Thanks for opening an issue and a PR! I made a slight change in your post so that it correctly links to the issue you created. 💖

I'll get this triaged for review! ⚡

[rachaelrenk]

Thank you for submitting this PR! I made a few tweaks to match the changes to our style guide, otherwise this looks good. ✨

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