chore: Integrate Backstage configurations and update Plausible domain

- Add and update configurations for Backstage integration in `.env.example` file
- Remove duplicate entry for `PLAUSIBLE_DATA_DOMAIN` in `.env.example`

Author: johnnyhuy

.env.example

@@ -1,8 +1,11 @@
+# GitHub
 GITHUB_TOKEN=
 
+# Backstage
+PLAUSIBLE_DATA_DOMAIN=backstage.localhost
+PLAUSIBLE_SOURCE_URL=http://plausible.localhost/js/script.js
+
 # Plausible
 BASE_URL=http://plausible.localhost
-PLAUSIBLE_DATA_DOMAIN=http://backstage.localhost
-PLAUSIBLE_SOURCE_URL=http://plausible.localhost/js/script.js
 SECRET_KEY_BASE=
 TOTP_VAULT_KEY=