chore(examples): move async example to apps (#1503)

Move async example to examples/apps/async as full project.
Simplify a little by removing the need for the github api token.

Co-authored-by: Orhun Parmaksız <orhunparmaksiz@gmail.com>

examples/apps/async-github/README.md

@@ -0,0 +1,9 @@
+# Async GitHub demo
+
+This example demonstrates how to use Ratatui with widgets that fetch data from GitHub API asynchronously.
+
+To run this demo:
+
+```shell
+cargo run -p async-github
+```