A full example of creating Relay environment in Relay Environment Pro…

…vider docs

Summary:
Current example just have this for the creating of the environment:

```
const Environment = createNewEnvironment();
```

And it's not immediately clear where to get that function or what this function even supposed to do.

This diff provides an example implementation of a basic "create environment" functionality.

Reviewed By: voideanvalue

Differential Revision: D48647447

fbshipit-source-id: db966b9d45c12878ba5ecd6bd88b4b71422a0afd

website/docs/api-reference/hooks/relay-environment-provider.md

@@ -16,14 +16,53 @@ This component is used to set a Relay environment in React Context. Usually, a *
 
 ```js
 const React = require('React');
+const {
+  Store,
+  RecordSource,
+  Environment,
+  Network,
+  Observable,
+} = require("relay-runtime");
 
 const {RelayEnvironmentProvider} = require('react-relay');
 
-const Environment = createNewEnvironment();
+/**
+ * Custom fetch function to handle GraphQL requests for a Relay environment.
+ *
+ * This function is responsible for sending GraphQL requests over the network and returning
+ * the response data. It can be customized to integrate with different network libraries or
+ * to add authentication headers as needed.
+ *
+ * @param {RequestParameters} params - The GraphQL request parameters to send to the server.
+ * @param {Variables} variables - Variables used in the GraphQL query.
+ */
+function fetchFunction(params, variables) {
+  const response = fetch("http://my-graphql/api", {
+    method: "POST",
+    headers: [["Content-Type", "application/json"]],
+    body: JSON.stringify({
+      query: params.text,
+      variables,
+    }),
+  });
+
+  return Observable.from(response.then((data) => data.json()));
+};
+
+/**
+ * Creates a new Relay environment instance for managing (fetching, storing) GraphQL data.
+ */
+function createEnvironment() {
+  const network = Network.create(fetchFunction);
+  const store = new Store(new RecordSource());
+  return new Environment({ store, network });
+}
+
+const environment = createEnvironment();
 
 function Root() {
   return (
-    <RelayEnvironmentProvider environment={Environment}>
+    <RelayEnvironmentProvider environment={environment}>
       <App />
     </RelayEnvironmentProvider>
   );