Guide updated in line with changes in rails commands [skip ci]

[soartec-lab]

Summary

The guide has also been updated as the rails commands change.

• Gemfiles generated by rails new do not include gems coffee-rails and uglifier
• The generate commands neither scaffold nor controller generate JavaScript files.
• The newly created Rails application does not include the app / assets / javascripts / application.js file by default.

[kaspth]

Thanks!

[rmacklin]

👋

I was reading this guide and noticed a bit of confusing wording on the two lines following this one (unfortunately GitHub won't let me comment directly on those lines). After looking through the history I found that the wording became confusing when this PR removed the mentions of uglifier and coffee-rails, leaving only sass-rails.

So, I've gone ahead and opened #38782 as a follow-up to address this.