fix: Remove additional resources section from Using Ruby on Rails for Your Backend lesson

[bittoby]

Because

The additional resources in the "Using Ruby on Rails for Your Backend" lesson have been evaluated and are no longer needed as part of the broader React additional resources cleanup initiative.

This PR

• Remove the entire "Additional resources" section from the Using Ruby on Rails for Your Backend lesson

Issue

Closes #30920

Additional Information

N/A

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