Update nuget docs to specify only .NET Framework projects are supported. (#35124)

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

.cspell.json

@@ -53,6 +53,7 @@
     "Jumpstart",
     "keyframes",
     "libera",
+    "libman",
     "Libsass",
     "lightboxes",
     "Lowercased",

site/content/docs/5.1/getting-started/download.md

@@ -108,7 +108,7 @@ composer require twbs/bootstrap:{{< param current_version >}}
 
 ### NuGet
 
-If you develop in .NET, you can also install and manage Bootstrap's [CSS](https://www.nuget.org/packages/bootstrap/) or [Sass](https://www.nuget.org/packages/bootstrap.sass/) and JavaScript using [NuGet](https://www.nuget.org/):
+If you develop in .NET Framework, you can also install and manage Bootstrap's [CSS](https://www.nuget.org/packages/bootstrap/) or [Sass](https://www.nuget.org/packages/bootstrap.sass/) and JavaScript using [NuGet](https://www.nuget.org/). Newer projects should use [libman](https://docs.microsoft.com/en-us/aspnet/core/client-side/libman/) or another method as NuGet is designed for compiled code, not frontend assets.
 
 ```powershell
 Install-Package bootstrap