docs: stub environment variables docs

docs/latest/viewer/environment-variables.md

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+# Environment Variables
+
+There are a number of environment variables we use at build time to influence the output application's behavior.
+
+```bash
+# Application
+NODE_ENV=< production | development >
+DEBUG=< true | false >
+APP_CONFIG=< relative path to application configuration file >
+PUBLIC_URL=<>
+VERSION_NUMBER=<Set by CircleCI>
+BUILD_NUM=<Set by CircleCI>
+# i18n
+USE_LOCIZE=<false>
+LOCIZE_PROJECTID=<ProjectID to pull translations for>
+LOCIZE_API_KEY=<To enable Locize live editing of translations>
+```
+
+## Setting Environment Variables
+
+- `npx cross-env`
+- `.env` files
+- env variables on build machine, or for terminal session