docs: add staging env examples for metadata settings

Author: joonsoome

.env.stage.example

@@ -15,7 +15,13 @@ WEB_LIBRARY_PORT=8281
 PDF_PROXY_BASE_URL=http://nas.local:8280/pdf
 
 # Optional: metadata source (used at build time for the Web Library image)
-ZOTERO_API_KEY=
+# If you host metadata on zotero.org (default):
+#   ZOTERO_API_KEY: your Zotero API key
+#   ZOTERO_API_AUTHORITY_PART: api.zotero.org
+# If you host metadata on-prem (custom API host):
+#   ZOTERO_API_KEY: on-prem API token (if required)
+#   ZOTERO_API_AUTHORITY_PART: e.g., nas.local:8080 (your on-prem metadata host)
+ZOTERO_API_KEY=changeme-zotero-api-key
 ZOTERO_API_AUTHORITY_PART=api.zotero.org
-ZOTERO_USER_SLUG=
-ZOTERO_USER_ID=
+ZOTERO_USER_SLUG=changeme-user-slug
+ZOTERO_USER_ID=1234567