📚 Add n8n documentation page

docs/sensitive-data.md

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+# Sensitive Data via File
+
+To avoid passing sensitive information via environment variables "_FILE" may be
+appended to some environment variables. It will then load the data from a file
+with the given name. That makes it possible to load data easily from
+Docker- and Kubernetes-Secrets.
+
+The following environment variables support file input:
+
+ - DB_MONGODB_CONNECTION_URL_FILE
+ - DB_POSTGRESDB_DATABASE_FILE
+ - DB_POSTGRESDB_HOST_FILE
+ - DB_POSTGRESDB_PASSWORD_FILE
+ - DB_POSTGRESDB_PORT_FILE
+ - DB_POSTGRESDB_USER_FILE
+ - N8N_BASIC_AUTH_PASSWORD_FILE
+ - N8N_BASIC_AUTH_USER_FILE