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.

Check off the following:

• [x] 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).
• [x] 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")

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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

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
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! ⚡

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