[chore] [Doc] Replace codesandbox links by stackblitz for issue reproduction

Author: slax57

.github/CONTRIBUTING.md

@@ -16,7 +16,7 @@ For code issues please include:
 * React-admin version
 * React version
 * Browser version
-* A code example or link to a repo, gist or running site. (hint: fork [this sandbox](https://codesandbox.io/s/github/marmelab/react-admin/tree/master/examples/simple) to create a reproducible version of your bug)
+* A code example or link to a repo, gist or running site. (hint: fork [this sandbox](https://stackblitz.com/github/marmelab/react-admin/tree/master/examples/simple) to create a reproducible version of your bug)
 
 For visual or layout problems, images or animated gifs can help explain your issue.
 It's even better with a live reproduction test case.

.github/ISSUE_TEMPLATE/Bug_report.md

@@ -21,11 +21,9 @@ about: Something isn't working as expected. Please tell us!
 **Related code:**
 <!-- If you are able to illustrate the bug or feature request with an example, please provide a sample application via one of the following means: -->
 
-* Preferably, a CodeSandbox forked from 
-  - https://codesandbox.io/s/github/marmelab/react-admin/tree/master/examples/simple (v4)
-  - https://codesandbox.io/s/github/marmelab/react-admin/tree/3.x/examples/simple (v3)
-* A StackBlitz forked from 
+* Preferably, a sandbox forked from 
   - https://stackblitz.com/github/marmelab/react-admin/tree/master/examples/simple (v4)
+  - https://codesandbox.io/s/github/marmelab/react-admin/tree/3.x/examples/simple (v3)
 * A link to a GitHub repo with the minimal codebase to reproduce the issue
 
 

examples/simple/README.md

@@ -1,6 +1,6 @@
 # React-admin Simple Example
 
-This is the application we use for our end-to-end tests, and for reproducing bugs via CodeSandbox.
+This is the application we use for our end-to-end tests, and for reproducing bugs via [Stackblitz](https://stackblitz.com/github/marmelab/react-admin/tree/master/examples/simple).
 
 ## How to run
 

packages/react-admin/README.md

@@ -139,7 +139,7 @@ React-admin is designed as a library of loosely coupled React components built o
 
 There are several examples inside the `examples` folder:
 
-* `simple` ([CodeSandbox](https://codesandbox.io/s/github/marmelab/react-admin/tree/master/examples/simple)): a simple application with posts, comments and users that we use for our e2e tests.
+* `simple` ([Stackblitz](https://stackblitz.com/github/marmelab/react-admin/tree/master/examples/simple)): a simple application with posts, comments and users that we use for our e2e tests.
 * `tutorial` ([CodeSandbox](https://codesandbox.io/s/github/marmelab/react-admin/tree/master/examples/tutorial)): the application built while following the tutorial.
 * `demo`: ([Live](https://marmelab.com/react-admin-demo/)) A fictional poster shop admin, serving as the official react-admin demo.