Add README.md files for multiple @kit packages

Added README.md files for several @kit packages including Supabase, UI, Emails, Billing, and Mailers. These documents explain the purpose and usage of each package, facilitating better understanding for developers interacting with them.

packages/billing/README.md

@@ -0,0 +1,13 @@
+# Billing - @kit/billing
+This package is responsible for managing billing and subscription related operations.
+
+Make sure the app installs the `@kit/billing` package before using it.
+
+```json
+{
+    "name": "my-app",
+    "dependencies": {
+        "@kit/billing": "*"
+    }
+}
+```