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

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+# 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": "*"
+    }
+}
+```

packages/emails/README.md

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+# Emails - @kit/emails
+
+This package is responsible for managing email templates using the react.email library.
+
+Here you can define email templates using React components and export them as a function that returns the email content.

packages/mailers/README.md

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+# Mailers - @kit/mailers
+
+This package is responsible for sending emails using a unified interface across the app.
+
+The default mailer uses the `nodemailer` package to send emails. You can create custom mailers by extending the `Mailer` class.
+
+Make sure the app installs the `@kit/mailers` package before using it.
+
+```json
+{
+    "name": "my-app",
+    "dependencies": {
+        "@kit/mailers": "*"
+    }
+}
+```
+
+## Usage
+
+### Send an email
+
+```javascript
+import { Mailer } from '@kit/mailers';
+
+const mailer = new Mailer();
+
+mailer.send({
+    to: '',
+    from: '',
+    subject: 'Hello',
+    text: 'Hello, World!'
+});
+```

packages/supabase/README.md

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+# Supabase - @kit/supabase
+
+This package is responsible for managing the Supabase client and various utilities related to Supabase.
+
+Make sure the app installs the `@kit/supabase` package before using it.
+
+```json
+{
+    "name": "my-app",
+    "dependencies": {
+        "@kit/supabase": "*"
+    }
+}
+```

packages/ui/README.md

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+# UI - @kit/ui
+
+This package is responsible for managing the UI components and styles across the app.
+
+This package define two sets of components:
+
+- `shadn-ui`: A set of UI components that can be used across the app using shadn UI
+- `makerkit`: Components specific to MakerKit