[docs] Fix the Gatsby example

Closes mui#11874

> Error: Module build failed: Error: Requires Babel "^7.0.0-0", but was loaded with "6.26.3". If you are sure you have a compatible version of @babel/core, it is likely that something in your build process is loading the wrong version. Inspect the stack trace of this error to look for the first entry that doesn't mention "@babel/core" or "babel-core" to see what is calling Babel.

examples/gatsby/.babelrc

@@ -0,0 +1 @@
+{}