fix: sentry init is using the wrong environment (#4940)

In the previous PR #4912 it seems that I forgot to pass the environment
on the backend.
Here is a quick fix!

I also added some "doc" in the the .env.example

packages/twenty-server/src/engine/integrations/environment/environment-variables.ts

@@ -235,7 +235,7 @@ export class EnvironmentVariables {
     (env) => env.EXCEPTION_HANDLER_DRIVER === ExceptionHandlerDriver.Sentry,
   )
   @IsString()
-  SENTRY_DSN_FRONT: string;
+  SENTRY_FRONT_DSN: string;
 
   @ValidateIf(
     (env) => env.EXCEPTION_HANDLER_DRIVER === ExceptionHandlerDriver.Sentry,