Migrate examples back to CodeSandbox

Author: erikras

docs/examples.md

@@ -8,19 +8,19 @@ Wanna help? We need to migrate all of these examples from CodeSandbox to [here](
 
 ---
 
-### [Simple Example](http://jinno.io/app/12/?source=react-final-form)
+### [Simple Example](examples/simple)
 
 Uses the built-in React inputs: `input`, `select`, and `textarea` to build a form with no validation.
 
-### [Synchronous Record-Level Validation](http://jinno.io/app/13/?source=react-final-form)
+### [Synchronous Record-Level Validation](examples/record-level-validation)
 
 Introduces a whole-record validation function and demonstrates how to display errors next to fields using child render functions.
 
-### [Synchronous Field-Level Validation](http://jinno.io/app/14/?source=react-final-form)
+### [Synchronous Field-Level Validation](examples/field-level-validation)
 
 Introduces field-level validation functions and demonstrates how to display errors next to fields using child render functions.
 
-### [Synchronous Record-Level Validation (with delayed error render)](https://jinno.io/app/15/?source=react-final-form)
+### [Synchronous Record-Level Validation (with delayed error render)](https://codesandbox.io/s/z2zqr008pm)
 
 Sometimes you want to give your user a chance to make it through a brief invalid value on their way to a valid one, e.g. a date string that needs two numbers on either side of a slash. With a simple delayed rendering component, this becomes easy. Plus, the error will disappear immediately when the user fixes the problem.