[docs] Redirect /r/discord to the support page

Author: brijeshb42

docs/public/_redirects

@@ -24,6 +24,8 @@
 /r/custom-component-variants /material-ui/customization/how-to-customize/#adding-new-component-variants 302
 /r/migration-v4 /material-ui/migration/migration-v4/ 302
 /r/pigment /blog/introducing-pigment-css/ 302
+# The Discord server is closed, point the stale links to the support page.
+/r/discord /material-ui/getting-started/support/ 302
 
 # Legacy redirection
 # Added in chronological order (the last line is the most recent one)