Update Asset Pipeline Guideline reference to Sass gem (rails#47491)

* Update Asset Pipeline guide refs to Sass gem

Sassc-rails gem is no longer maintained. Sass is now compiled by Dart. The new dartsass-rails gem wraps the standalone release of the Dart version of Sass.

* Update asset_pipeline.md

---------

Co-authored-by: David Heinemeier Hansson <[email protected]>

Author: leesheppard

guides/source/asset_pipeline.md

@@ -33,11 +33,16 @@ passing the `--skip-asset-pipeline` option.
 $ rails new appname --skip-asset-pipeline
 ```
 
-Rails can easily work with Sass by adding the [`sassc-rails`](https://github.com/sass/sassc-rails)
-gem to your `Gemfile`, which is used by Sprockets for [Sass](https://sass-lang.com) compilation:
+Rails can easily work with [Sass](https://sass-lang.com) by adding the [`dartsass-rails`](https://github.com/rails/dartsass-rails) gem to your `Gemfile`:
 
 ```ruby
-gem 'sassc-rails'
+gem 'dartsass-rails'
+```
+
+After bundling the gem, run the following to complete the installation:
+
+```bash
+$ rails dartsass:install
 ```
 
 To set asset compression methods, set the appropriate configuration options
@@ -49,9 +54,6 @@ config.assets.css_compressor = :yui
 config.assets.js_compressor = :terser
 ```
 
-NOTE: The `sassc-rails` gem is automatically used for CSS compression if included
-in the `Gemfile` and no `config.assets.css_compressor` option is set.
-
 [`config.assets.css_compressor`]: configuring.html#config-assets-css-compressor
 [`config.assets.js_compressor`]: configuring.html#config-assets-js-compressor