Add clarifying documentation for how to add a sass stylesheet on Windows (#6322)

Author: monish001

docusaurus/docs/adding-a-sass-stylesheet.md

@@ -42,6 +42,12 @@ If you set `SASS_PATH=node_modules:src`, this will allow you to do imports like
 @import 'nprogress/nprogress'; // importing a css file from the nprogress node module
 ```
 
+> **Note:** For windows operating system, use below syntax
+>
+> ```
+> SASS_PATH=./node_modules;./src
+> ```
+
 > **Tip:** You can opt into using this feature with [CSS modules](adding-a-css-modules-stylesheet.md) too!
 
 > **Note:** If you're using Flow, override the [module.file_ext](https://flow.org/en/docs/config/options/#toc-module-file-ext-string) setting in your `.flowconfig` so it'll recognize `.sass` or `.scss` files. You will also need to include the `module.file_ext` default settings for `.js`, `.jsx`, `.mjs` and `.json` files.