docs: add index query param doc (#13900)

Author: brookslybrand

docs/explanation/index-query-param.md

@@ -0,0 +1,86 @@
+---
+title: Index Query Param
+---
+
+# Index Query Param
+
+[MODES: framework, data]
+
+## Overview
+
+You may find a wild `?index` appearing in the URL of your app when submitting forms.
+
+Because of nested routes, multiple routes in your route hierarchy can match the URL. Unlike navigations where all matching route [`loader`][loader]s are called to build up the UI, when a [`form`][form_element] is submitted, _only one action is called_.
+
+Because index routes share the same URL as their parent, the `?index` param lets you disambiguate between the two.
+
+## Understanding Index Routes
+
+For example, consider the following route structure:
+
+```ts filename=app/routes.ts
+import {
+  type RouteConfig,
+  route,
+  index,
+} from "@react-router/dev/routes";
+
+export default [
+  route("projects", "./pages/projects.tsx", [
+    index("./pages/projects/index.tsx"),
+    route(":id", "./pages/projects/project.tsx"),
+  ]),
+] satisfies RouteConfig;
+```
+
+This creates two routes that match `/projects`:
+
+- The parent route (`./pages/projects.tsx`)
+- The index route (`./pages/projects/index.tsx`)
+
+## Form Submission Targeting
+
+For example, consider the following forms:
+
+```tsx
+<Form method="post" action="/projects" />
+<Form method="post" action="/projects?index" />
+```
+
+The `?index` param will submit to the index route; the action without the index param will submit to the parent route.
+
+When a [`<Form>`][form_component] is rendered in an index route without an [`action`][action], the `?index` param will automatically be appended so that the form posts to the index route. The following form, when submitted, will post to `/projects?index` because it is rendered in the context of the `projects` index route:
+
+```tsx filename=app/pages/projects/index.tsx
+function ProjectsIndex() {
+  return <Form method="post" />;
+}
+```
+
+If you moved the code to the project layout (`./pages/projects.tsx` in this example), it would instead post to `/projects`.
+
+This applies to `<Form>` and all of its cousins:
+
+```tsx
+function Component() {
+  const submit = useSubmit();
+  submit({}, { action: "/projects" });
+  submit({}, { action: "/projects?index" });
+}
+```
+
+```tsx
+function Component() {
+  const fetcher = useFetcher();
+  fetcher.submit({}, { action: "/projects" });
+  fetcher.submit({}, { action: "/projects?index" });
+  <fetcher.Form action="/projects" />;
+  <fetcher.Form action="/projects?index" />;
+  <fetcher.Form />; // defaults to the route in context
+}
+```
+
+[loader]: ../api/data-routers/loader
+[form_element]: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/form
+[form_component]: ../api/components/Form
+[action]: ../api/data-routers/action