[docs] Fix 414 HTTP issue (#9635)

We have done quite some iteration on this issue.
We should be in a good shape now.
I think that it would be good in the future to add a link
toward a different online editor.

@lukePeavey I haven't ported your image replacement logic.
I couldn't find it. Feel free to submit a pull-request
to add it in the future :).

Closes #9474

docs/src/modules/components/Demo.js

@@ -139,7 +139,7 @@ if (rootElement) {
             ref={node => {
               this.codesandboxForm = node;
             }}
-            method="get"
+            method="post"
             action="https://codesandbox.io/api/v1/sandboxes/define"
             target="_blank"
           >