chore(docs): Restore dynamic function routing docs (#33641)

We removed them in #31809 as the cloud implementation wasn't
yet done. But now it is!

docs/docs/reference/functions/getting-started.md

@@ -34,6 +34,13 @@ A Function file must export a single function that takes two parameters:
 - `req`: Node's [http request object](https://nodejs.org/api/http.html#http_class_http_incomingmessage) with some [automatically parsed data](/docs/reference/functions/getting-started/#common-data-formats-are-automatically-parsed)
 - `res`: Node's [http response object](https://nodejs.org/api/http.html#http_class_http_serverresponse) with some [extra helper functions](/docs/reference/functions/middleware-and-helpers/#res-helpers)
 
+Dynamic routing is supported for creating REST-ful APIs and other uses cases
+
+- `/api/users` => `src/api/users/index.js`
+- `/api/users/23` => `src/api/users/[id].js`
+
+[Learn more about dynamic routes.](/docs/reference/functions/routing#dynamic-routing)
+
 ## Typescript
 
 Functions can be written in JavaScript or Typescript.
@@ -202,5 +209,4 @@ Shadowing with functions works similar to how shadowing works in general. You ca
 
 ## Limitations
 
-- Gatsby Functions do not support dynamic routes in Gatsby Cloud at the moment
 - Bundling in native dependencies is not supported at the moment

docs/docs/reference/functions/routing.md

@@ -4,9 +4,45 @@ title: Routing
 
 Function routing shares the same syntax as [page routing](/docs/reference/routing/file-system-route-api/).
 
+## Static Routing
+
 Both top-level and nested routes are supported.
 
 - `src/api/top-level.js` => `/api/top-level`
 - `src/api/directory/foo.js` => `/api/directory/foo`
 
 `index.js` files are routed at their directory path e.g. `src/api/users/index.js` => `/api/users`
+
+## Dynamic Routing
+
+_Note: Dynamic Routing is not yet supported on Gatsby Cloud. Expect it in another few weeks. It will work locally._
+
+### Param routes
+
+Use square brackets (`[ ]`) in the file path to mark dynamic segments of the URL.
+
+So to create an Function for fetching user information by `userId`:
+
+```js:title=src/api/users/[id].js
+export default async function handler(req, res) {
+  const userId = req.params.id
+  // Fetch user
+  const user = await getUser(userId)
+  res.json(user)
+}
+```
+
+Dynamic routes share syntax with [client-only routes](/docs/reference/routing/file-system-route-api/#creating-client-only-routes).
+
+### Splat routes
+
+Gatsby also supports splat (or wildcard) routes, which are routes that will match anything after the splat. These are less common, but still have use cases.
+
+```js:title=src/api/foo/[...].js
+export default function handler(req, res) {
+  const params = req.params[`*`].split(`/`)
+
+  // `src/api/foo/1/2 // params[0] === `1`
+  // params[1] === `2`
+}
+```