[docs] Point to material-ui-x/next instead of master

Author: oliviertassinari

docs/public/_redirects

@@ -146,8 +146,8 @@ https://v3-9-0.material-ui.com/* https://v3.material-ui.com/:splat 301!
 ## material-ui-x
 ## Unlike the store that expect to be hosted under a subfolder,
 ## material-ui-x is configured to be hosted at the root.
-/api/*/ https://material-ui-x.netlify.app/api/:splat/ 200
-/components/* https://material-ui-x.netlify.app/components/:splat 200
+/api/*/ https://next--material-ui-x.netlify.app/api/:splat/ 200
+/components/* https://next--material-ui-x.netlify.app/components/:splat 200
 /components/data-grid/demo/ https://muix-preview.netlify.app/ 200
-/_next/* https://material-ui-x.netlify.app/_next/:splat 200
-/static/x/* https://material-ui-x.netlify.app/static/x/:splat 200
+/_next/* https://next--material-ui-x.netlify.app/_next/:splat 200
+/static/x/* https://next--material-ui-x.netlify.app/static/x/:splat 200