1774 extensibility v1 create an exhaustive documentation readme or dedicated section in twenty contributing doc (#16751)

As title

<img width="1108" height="894" alt="image"
src="https://github.com/user-attachments/assets/e2dc7e12-72e3-4ca3-ac7b-a94de547f82a"
/>

packages/twenty-sdk/src/cli/commands/app.command.ts

@@ -106,7 +106,7 @@ export class AppCommand {
       });
 
     appCommand
-      .command('generate [outputPath]')
+      .command('generate [appPath]')
       .description('Generate Twenty client')
       .action(async (appPath?: string) => {
         await this.generateCommand.execute(formatPath(appPath));