docs: dotenvx workflow for env files

.env.example moves to the repository root with dotenvx setup/run notes; the
README documents 'dotenvx run -f .env -- node tools/server.js' and the
Docker caveat (--env-file cannot read encrypted values).

tools/.env.example

@@ -1,9 +0,0 @@
-# Image Cropper service credentials — the ONLY environment variables this app
-# uses. Everything else (URLs, port, domain catalog) is hardcoded in source.
-# Without these the rest of the app works and /cropper answers 503.
-MINIO_ACCESS_KEY=
-MINIO_SECRET_KEY=
-AZURE_STORAGE_CONNECTION_STRING=
-NODERED_HTTP_USERNAME=
-NODERED_HTTP_PASSWORD=
-CARVEKIT_API_KEY=