Docs: add alternative CDNs section in Getting Started > Download (#39145)

Co-authored-by: Mark Otto <markd.otto@gmail.com>

Author: julien-deramond

site/content/docs/5.3/getting-started/download.md

@@ -50,6 +50,14 @@ If you're using our compiled JavaScript and prefer to include Popper separately,
 <script src="{{< param "cdn.js" >}}" integrity="{{< param "cdn.js_hash" >}}" crossorigin="anonymous"></script>
 ```
 
+### Alternative CDNs
+
+We recommend [jsDelivr](https://www.jsdelivr.com/) and use it ourselves in our documentation. However, in some cases—like in some specific countries or environments—you may need to use other CDN providers like [cdnjs](https://cdnjs.com/) or [unpkg](https://unpkg.com/).
+
+You'll find the same files on these CDN providers, albeit with different URLs. When changing the URLs, you'll also need to update the `integrity` attribute. Tools like [SRI Hash Generator](https://www.srihash.org/) can help you generate the correct values.
+
+With cdnjs, you can [use this direct Bootstrap package link](https://cdnjs.com/libraries/bootstrap) to copy and paste ready-to-use HTML snippets for each dist file from any version of Bootstrap.
+
 ## Package managers
 
 Pull in Bootstrap's **source files** into nearly any project with some of the most popular package managers. No matter the package manager, Bootstrap will **require a [Sass compiler]({{< docsref "/getting-started/contribute#sass" >}}) and [Autoprefixer](https://github.com/postcss/autoprefixer)** for a setup that matches our official compiled versions.