Review and update instructions for making content edits in GitHub

[Katherine-MN]

Issue description

We need to revisit our documentation on how to make content updates in GitHub. The goal is to ensure it's still accurate and easy to follow, regardless of familiarity with GitHub.

Acceptance criteria

• Evaluate current instructions for updating and maintaining content in GitHub.
• Determine where that content should live long term. i.e., new tab in the content guide vs. a new document
• Make updates to improve clarity and ease of contribution and consider adding screenshots
• Review updated document with the design team to see if there are gaps in understanding

Additional context

Note: The content guide—as a whole—will be evaluated in a separate ticket, so this ticket should only concentrate on reviewing/revising the sections related to updates in GitHub.