Rails: JS Bundling: update instructions for migrating away from importmaps

[JoshDevHub]

Because

The process for converting an importmap app to js bundling was inaccurate.

This PR

• Updates the instructions for converting an importmaps app over to using a JavaScript bundler
• Fixes various lint issues

Issue

Closes #26658

Additional Information

One additional point of consideration: the flow of the lesson just walks you through getting the JS to bundle correctly after starting a brand new app with importmaps and changing to a bundler. If a learner wanted to actually do this with an already functioning importmaps project, they'd have to do a couple of more things for it to work properly. Do we want to mention this or just leave it like it is?

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] 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
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [x] If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• [x] If any lesson files are included in this PR, they follow the Layout Style Guide

[JoshDevHub]

Also this is failing Markdownlint because of a missing assignment section. Addressing that seems like it'd require a larger structural change to the lesson, so I didn't want to do it in this PR.