docs: Direct DB Connection configuration

docs/sources/installation/configuration-sql.md

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+# SQL Data Source Configuration
+
+In order to use _Direct DB Connection_ feature you should configure SQL data source first.
+
+![Configure MySQL data source](../img/installation-mysql_ds_config.png)
+
+Select _MySQL_ data source type and provide your database host address and port (3306 is default for MySQL). Fill
+database name (usually, `zabbix`) and specify credentials.
+
+## Security notes
+
+As you can see in _User Permission_ note, Grafana doesn't restrict any queries to the database. So you should be careful
+and create a special user with limited access to Zabbix database. Grafana-Zabbix plugin uses only `SELECT` queries to
+`history`, `history_uint`, `trends` and `trends_uint` tables. So it's reasonable to grant only SELECT privileges to
+these tables for grafana user. But if you want to use this MySQL data source for querying another data, you can
+grant SELECT privileges to entire zabbix database.
+
+Also, all queries are invoked by grafana-server, so you can restrict connection to only grafana host.
+
+```sql
+GRANT SELECT ON zabbix.* TO 'grafana'@'grafana-host' identified by 'password';
+```