docs(how-to/spa): add more details about generating the index.html (#12592)

* Update spa.md to include more details about generating the index.html

* Update docs/how-to/spa.md

---------

Co-authored-by: Brooks Lybrand <[email protected]>

Author: alexanderson1993

docs/how-to/spa.md

@@ -71,4 +71,4 @@ In contrast `react-router build` (with server rendering disabled) pre-renders yo
 - Use React components to generate the initial page users see
 - Re-enable server rendering later without changing anything about your UI
 
-This is also why your project still needs a dependency on `@react-router/node`.
+React Router will still server render your index route to generate that `index.html` file. This is why your project still needs a dependency on `@react-router/node` and your routes need to be SSR-safe. That means you can't call `window` or other browser-only APIs during the initial render, even when server rendering is disabled.