Fix confusing wording in Asset Pipeline guide [ci skip]

[rmacklin]

Summary

While looking into #38307, I noticed some confusing wording in the Asset Pipeline guide:

rails/guides/source/asset_pipeline.md

Lines 36 to 45 in aeac447

	Rails automatically adds the `sass-rails` gem to your `Gemfile`, which is used
	by Sprockets for asset compression:
	```ruby
	gem 'sass-rails'
	```
	Using the `--skip-sprockets` option will prevent Rails from adding
	them to your `Gemfile`, so if you later want to enable
	the asset pipeline you will have to add those gems to your `Gemfile`. Also,

Specifically

adding them to your Gemfile

and

add those gems

is confusing because the preceding paragraph only mentions one gem (sass-rails). I discovered that this is a result of removing mentions of uglifier and coffee-rails from that paragraph in #35994.

So I've opened this PR as a follow-up to #35994 to clean up the confusing wording that was left in the guide.

Other Information

While I was in there, I also clarified the purpose of the sass-rails gem and added links to https://github.com/rails/sass-rails and https://sass-lang.com.

[eugeneius]

There are a few more outdated references in the Preprocessing section, in particular the assertion that Rails generates a CoffeeScript file with every scaffold by default. Could you do a pass over those too?

[rmacklin]

Sounds good, I've updated the Preprocessing section in e71692f

[eugeneius]

The sass gem is no longer maintained, and sass-rails now depends on the sassc gem instead via sassc-rails (#32896).

Even if it's not technically accurate, I think this should say "provided by the sass-rails gem" - for the purposes of following this guide, it's enough to know that the processor is available because sass-rails is in the Gemfile, even if the code is ultimately provided by a different gem.

[rmacklin]

👍 Updated to sass-rails in 649d265

[eugeneius]

Could you squash your commits? Otherwise I think this is ready to go.

[rmacklin]

Can you do a squash-and-merge? Or do you want me to squash into more than 1 commit?

[eugeneius]

If I use the squash and merge button, the autogenerated commit message will be enormous:

Fix confusing wording in Asset Pipeline guide [ci skip]

* Clean up leftover plural references in Asset Pipeline guide

Prior to ab123a33d2460dcdc5c36001cef5316eadc75fb3, this guide mentioned
that three gems (sass-rails, uglifier, and coffee-rails) would be added
to the Gemfile by default, unless the `--skip-sprockets` option was
used when generating the application.
However, ab123a33d2460dcdc5c36001cef5316eadc75fb3 removed uglifier and
coffee-rails from the guide (to reflect the switch to webpacker for
managing javascript in rails 6). Now that only sass-rails is left, the
guide should say "adding this to your Gemfile" instead of "adding them
to your Gemfile" and "add that gem" instead of "add those gems".

[ci skip]

* Clarify what sass-rails is used for in Asset Pipeline guide

The original text "used by Sprockets for asset compression" was written
when this sentence was summarizing the purpose of three gems:
sass-rails, uglifier, and coffee-rails.
However, ab123a33d2460dcdc5c36001cef5316eadc75fb3 removed uglifier and
coffee-rails from this sentence. Now that only sass-rails is left, we
can be more specific about what it is used for: compiling Sass
stylesheets.

[ci skip]

* Link to the sass-rails repo and sass-lang.com in Asset Pipeline guide

We link to the sprockets-rails repository in the preceding paragraph,
so it seems appropriate to also link to the sass-rails repository in
this paragraph. And since readers of this guide may not know what Sass
is, it also seems appropriate to link to the sass-lang.com website.

[ci skip]

* Further reword Asset Pipeline guide introduction to be less repetitive

[ci skip]

Co-Authored-By: Eugene Kenny <elkenny@gmail.com>

* Remove mention of generating CoffeeScript files in Preprocessing section

of the Asset Pipeline guide

This section previously declared that the Rails controller generator
generated an `app/assets/javascripts/projects.coffee` and an
`app/assets/stylesheets/projects.scss` file. Then, in
ab123a33d2460dcdc5c36001cef5316eadc75fb3, we removed the assertion that
`app/assets/javascripts/projects.coffee` would be generated (to reflect
the switch to webpacker for managing javascript in Rails 6). However,
the preceding sentence and the subsequent paragraph were still written
with the assumption that a CoffeeScript file would be generated.

Now we've reworded those parts to be consistent with the fact that Rails
no longer generates a CoffeeScript file.

[ci skip]

* Remove mention of the deprecated `sass` gem from Asset Pipeline guide

As of https://github.com/rails/sass-rails/commit/409d871c2a971691eac86c5a32e614e2941c72b8
sass-rails no longer uses the `sass` gem (which is no longer
maintained; it has been replaced by the `sassc` gem). Thus, the rails
guides should not mention the `sass` gem specifically. Instead, they can
continue to reference the `sass-rails` gem.

[ci skip]

Co-authored-by: Eugene Kenny <elkenny@gmail.com>

This is way too much detail for the patch it describes. I could edit it down myself, but I'd prefer if you handled it by squashing the commits and writing a shorter combined commit message.

[rmacklin]

I can squash and push later when I return to my rails dev machine. (Unfortunately, recloning on my current connection is not feasible.)

Alternatively, if you want to squash-merge now, here's a proposal for an abbreviated commit message:

Fix confusing wording in Asset Pipeline guide

* Clean up leftover plural "gems" references as a follow-up to
  ab123a33d2460dcdc5c36001cef5316eadc75fb3 (#38307)
* Remove mention of generating CoffeeScript files in Preprocessing section
* Clarify what sass-rails is used for
* Link to the sass-rails repo and sass-lang.com
* Remove mention of the deprecated `sass` gem to reflect the change from
  https://github.com/rails/sass-rails/commit/409d871c2a971691eac86c5a32e614e2941c72b8

[ci skip]

[eugeneius]

Perfect, thanks! 💛 I squash-merged with that message.