Update "Configuring Git for GitHub Desktop" article

[steveward]

Why:

Closes#4033

What's being changed:

• Updated the "Configuring Git for GitHub Desktop" article with the new dropdown option for selecting an email address to use for committing
• Added a section describing how to set an email address for commits on a per-repository basis
• Updated a bunch of screenshots and created new ones where needed
• Updated and renamed 'paste-email-git-config' to 'select-email-git-config' reusable since you can now use a dropdown to select an email address

Check off the following:

• I have reviewed my changes in staging (look for "Automatically generated comment" and click Modified 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")

[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
desktop/contributing-and-collaborating-using-github-desktop/working-with-your-remote-repository-on-github-or-github-enterprise/changing-a-remotes-url-from-github-desktop.md	fpt	fpt
desktop/installing-and-configuring-github-desktop/configuring-and-customizing-github-desktop/configuring-git-for-github-desktop.md	fpt	fpt

[jkidd11]

assets/images/help/desktop/mac-select-default-branch-name.png

[lecoursen]

This is looking great @steveward! ✨ In one of my comments I asked you to add another brief paragraph, and I'll give this another quick review after you do!

[steveward]

@lecoursen this should be ready for another review! Thanks!

[lecoursen]

This looks so great @steveward! 🎉:sparkles: I left two small suggestions to consider, but this looks ready to merge after you 👀 those.

[github-actions]

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