Add docs on polyfilling fetch

Author: brophdawg11

docs/deploying/custom-node.md

@@ -7,3 +7,41 @@ title: Custom Node.js
 <docs-warning>
   This document is a work in progress. There's not much to see here (yet).
 </docs-warning>
+
+## Polyfilling `fetch`
+
+React Router officially supports Active and Maintenance[^1] [Node LTS veleases][node-releases] at any given point in time. Dropping support for End of Life Node versions may be done in a React Router Minor release.
+
+[^1] Based on timing, React Router may drop support for a Node Maintenance LTS version shortly before it goes end-of-life if it better aligns with a React Router Major SemVer release.
+
+At the time React Router v7 was released, all versions had a usable `fetch` implementation so there is generally no need to polyfill any `fetch` APIs so long as you're on Node 22 or one of the later Node 20 releases.
+
+- Node 22 (Active LTS) has a stable [`fetch`][node-22-fetch] implementation
+- Node 20 (Maintenance LTS) has an experimental (but suitable from our testing) [`fetch`][node-20-fetch] implementation
+
+If you do find that you need to polyfill anything, you can do so directly from the [undici] package which node uses internally.
+
+```ts
+import {
+  fetch as nodeFetch,
+  File as NodeFile,
+  FormData as NodeFormData,
+  Headers as NodeHeaders,
+  Request as NodeRequest,
+  Response as NodeResponse,
+} from "undici";
+
+export function polyfillFetch() {
+  global.File = NodeFile;
+  global.Headers = NodeHeaders;
+  global.Request = NodeRequest;
+  global.Response = NodeResponse;
+  global.fetch = nodeFetch;
+  global.FormData = NodeFormData;
+}
+```
+
+[node-releases]: https://nodejs.org/en/about/previous-releases
+[node-20-fetch]: https://nodejs.org/docs/latest-v20.x/api/globals.html#fetch
+[node-22-fetch]: https://nodejs.org/docs/latest-v22.x/api/globals.html#fetch
+[undici]: https://github.com/nodejs/undici