chore(docs): Updated with Windows Configuration (#20361)

* Updated with Windows Configuration

Added SET GATSBY_GRAPHQL_IDE=playground&gatsby develop as the configuration needed to make it work in Windows.

* Changed SET for cross-env

* Update docs/docs/using-graphql-playground.md

Tried it out and worked too

Co-Authored-By: Vladimir Razuvaev <vladimir.razuvaev@gmail.com>

Co-authored-by: LB <laurie@gatsbyjs.com>
Co-authored-by: Vladimir Razuvaev <vladimir.razuvaev@gmail.com>

docs/docs/using-graphql-playground.md

@@ -18,6 +18,18 @@ To access this experimental feature utilizing GraphQL Playground with Gatsby, ad
 "develop": "GATSBY_GRAPHQL_IDE=playground gatsby develop",
 ```
 
+If you are on **Windows** then you should use:
+
+```
+"develop": "cross-env GATSBY_GRAPHQL_IDE=playground gatsby develop",
+```
+
+If you haven't installed **cross-env**, you can do it with:
+
+```
+npm install --save-dev cross-env
+```
+
 Use `npm run develop` instead of `gatsby develop` and access it when the development server is running on `http://localhost:8000/___graphql`
 
 To still be able to use `gatsby develop` you would require the dotenv package to your gatsby-config.js file and add an [environment variable](/docs/environment-variables/) file, typically called `.env.development`. Finally, add `GATSBY_GRAPHQL_IDE=playground` to the `.env.development` file.