Linting and RuboCop: Update extension installation instructions

[SumonGFC]

Because

The lesson "Linting and RuboCop" contained instructions on how to install RuboCop extensions, which confused certain members in a discord conversation.

This PR

• Makes explicit what a RuboCop extension is (I.e. a Gem)
• Makes explicit the necessary steps to install and setup a RuboCop extension
• Re-phrases certain sections of the lesson to accommodate the material added by the previous to bullet points

Issue

Closes #28269

Additional Information

Discord discussion

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