Foundations/Installations/Setting Up Git: Add note about extra GH keys

[SumonGFC]

Because

Some users in discord asked for help on what to do if they have set up GH ssh keys on another machine. This PR adds a note about what to do in this situation.

This PR

• adds lesson note about what to do to if a user already has set up GH ssh keys on another machine prior to starting TOP

Issue

Closes #28716

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide

[SumonGFC]

I was somewhat indecisive about where to place the lesson note. @ reviewer let me know if you think it could be placed somewhere better.

[wise-king-sullyman]

Looks great, thanks!