Clarifies Sass variable defaults, adds bootstrap-npm-starter callout (#32795)

* Clarify Sass import and customize docs for how to modify variable defaulst

* Add an npm starter project callout to a few pages

* Update callout-info-npm-starter.md

Co-authored-by: XhmikosR <xhmikosr@gmail.com>

site/content/docs/5.0/getting-started/build-tools.md

@@ -65,6 +65,10 @@ Our [package.json]({{< param repo >}}/blob/v{{< param current_version >}}/packag
   </tbody>
 </table>
 
+{{< callout info >}}
+{{< partial "callout-info-npm-starter.md" >}}
+{{< /callout >}}
+
 ## Sass
 
 Bootstrap uses [Dart Sass](https://sass-lang.com/dart-sass) for compiling our Sass source files into CSS files (included in our build process), and we recommend you do the same if you're compiling Sass using your own asset pipeline. We previously used Node Sass for Bootstrap v4, but LibSass and packages built on top of it, including Node Sass, are now [deprecated](https://sass-lang.com/blog/libsass-is-deprecated).

site/content/docs/5.0/getting-started/download.md

@@ -70,6 +70,10 @@ Bootstrap's `package.json` contains some additional metadata under the following
 - `sass` - path to Bootstrap's main [Sass](https://sass-lang.com/) source file
 - `style` - path to Bootstrap's non-minified CSS that's been precompiled using the default settings (no customization)
 
+{{< callout info >}}
+{{< partial "callout-info-npm-starter.md" >}}
+{{< /callout >}}
+
 ### yarn
 
 Install Bootstrap in your Node.js powered apps with [the yarn package](https://yarnpkg.com/en/package/bootstrap):