Add docs for double underscore cursor

[rhorber]

This adds the documentation for the cursor doubleUnderscore, as added in microsoft/terminal#7827.

[opbld33]

Docs Build status updates of commit 009e3a7:

✅ Validation status: passed

File	Status	Preview URL	Details
TerminalDocs/customize-settings/profile-settings.md	✅Succeeded	View

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