Add note about Figaro and Rails credentials to env variables lesson

[GodSpeed313]

Because

This PR

Issue

Closes #XXXXX

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

• Because

As discussed in Issue #30950, students transitioning from the Node.js path to Ruby on Rails often look for a familiar way to manage environment variables. This addition provides a "Looking Ahead" roadmap to bridge that gap.

This PR

• Adds a new section to the end of the Environment Variables lesson.
• Introduces the Figaro gem as a beginner-friendly alternative to .env.
• Briefly mentions Rails Credentials to give students a heads-up on built-in Rails security.
• Reinforces the core concept of Separation of Concerns.

Issue

Related to #30950

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
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes
• If any lesson files are included, they follow the Layout Style Guide

[mao-sz]

Closing this because

1. The desired work has not been made clear by a maintainer yet
2. No work has been assigned for the issue
3. Even if work was decided on and you were assigned, we have a lesson style guide that this does not conform to

Please familiarise yourself with our contributing guide (as well as the general contributing guide it links to). Please do not open PRs to close issues if you have not been assigned for specific work.