[docs] Add env var load order (#32350)

My team & I keep asking the question "What order are env vars _actually_ loaded in?".

This addition surfaces the order in a clear and readable way without having to read and understand the entire "Environment Variables" documentation first.

## Documentation / Examples

- [x] Make sure the linting passes by running `yarn lint`


Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

docs/basic-features/environment-variables.md

@@ -149,3 +149,29 @@ export default async () => {
   loadEnvConfig(projectDir)
 }
 ```
+
+## Environment Variable Load Order
+
+Depending on the environment (as set by `NODE_ENV`), variables are loaded from the following sources in order from top-to-bottom. In all environments existing env is not overridden by following sources.
+
+`NODE_ENV=production`
+
+- `.env.production.local`
+- `.env.local`
+- `.env.production`
+- `.env`
+
+`NODE_ENV=development`
+
+- `.env.development.local`
+- `.env.local`
+- `.env.development`
+- `.env`
+
+`NODE_ENV=test`
+
+- `.env.test.local`
+- `.env.test`
+- `.env`
+
+_(note: `.env.local` is not loaded when `NODE_ENV=test`)_