docs (README): adds a banner to the README

Adds a banner image with a link to the Be API website to the README file.
This improves visibility and branding.

Author: firestar300

.github/banner-github.png

@@ -1,3 +1,5 @@
+[![Be API Github Banner](.github/banner-github.png)](https://beapi.fr)
+
 # Blockparty Tabs
 
 An accessible tabs block for WordPress Gutenberg editor that follows ARIA best practices.