docs: add section on multiple layouts to gatsby-layout-plugin docs (#…

…9538)

Adding a section on how to use multiple layouts to gatsby-layout-plugin docs. Might be helpful to anyone trying to build a site with a more complex layout system (and who still needs to use or prefers the layout philosophy used in Gatsby v1).

packages/gatsby-plugin-layout/README.md

@@ -179,3 +179,33 @@ const ComponentThatChangeState = () => (
   </ContextConsumer>
 )
 ```
+
+### Handling multiple layouts
+
+If you want to use different layouts for different pages, you can pass this information in the context of the pages you create, and then conditionally render in your layout file.
+
+In `gatsby-node.js`:
+
+```js
+exports.onCreatePage = ({ page, actions }) => {
+  const { createPage } = actions
+
+  if(page.path.match(/special-page/) {
+    page.context.layout = 'special'
+    createPage(page)
+  }
+}
+```
+
+And then in `src/layouts/index.js`:
+
+```js
+export default ({ children, pageContext }) => {
+  if (pageContext.layout === 'special') {
+    return <AlternativeLayout>{children}</AlternativeLayout>
+  }
+  return (
+    <RegularLayout>{children}</RegularLayout>
+  ) 
+}
+```