Add an npm starter project callout to a few pages

site/content/docs/5.0/customize/sass.md

@@ -99,6 +99,10 @@ $body-color: #111;
 
 Repeat as necessary for any variable in Bootstrap, including the global options below.
 
+{{< callout info >}}
+{{< partial "callout-info-npm-starter.md" >}}
+{{< /callout >}}
+
 ## Maps and loops
 
 Bootstrap includes a handful of Sass maps, key value pairs that make it easier to generate families of related CSS. We use Sass maps for our colors, grid breakpoints, and more. Just like Sass variables, all Sass maps include the `!default` flag and can be overridden and extended.

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):

site/layouts/partials/callout-info-npm-starter.md

@@ -0,0 +1 @@
+**Get started with Bootstrap via npm with our starter project!** Head to the [twbs/bootstrap-npm-starter](https://github.com/twbs/bootstrap-npm-starter) template repository to see how to build and customize Bootstrap in your own npm project. Includes Sass compiler, Autoprefixer, Stylelint, Purge CSS, and Bootstrap Icons.