docs: crosslink web sections (#87)

Author: vonovak

docs/setting-up/web.mdx

@@ -68,3 +68,7 @@ After the script is loaded, you can call the functions for signing in and render
 If you call any of the module functions before the client library is loaded, such calls trigger the [`onError` callback](/docs/api#webonetapsignincallbacks) with the [`PLAY_SERVICES_NOT_AVAILABLE` error code](../errors#status-codes).
 
 You can read the official docs for loading the Client Library [here](https://developers.google.com/identity/gsi/web/guides/client-library).
+
+## Usage
+
+See [here](../web-support#usage).

docs/web-support.mdx

@@ -20,6 +20,10 @@ On the web, the `signIn` and `createAccount` functions are not `Promise`-based b
 The reason for callback-based apis rather than promise-based is that it's possible to get into an "error" state (when one-tap is not available) and later get a successful sign in from the button flow. Because of how the Google Sign In for web SDK is done, modeling this with a promise-based api is not possible.
 :::
 
+## Setup
+
+See [here](setting-up/web).
+
 ## Usage
 
 To implement web support, follow these two steps: