docs(config): add missing QuestDB link (cube-js#5308)

Author: hassankhan

docs/content/Configuration/Databases/QuestDB.mdx

@@ -33,3 +33,5 @@ CUBEJS_DB_PASS=quest
 | `CUBEJS_DB_PASS`     | The password used to connect to the database                                        | A valid database password |    ✅    |
 | `CUBEJS_CONCURRENCY` | The number of concurrent connections each queue has to the database. Default is `2` | A valid number            |    ❌    |
 | `CUBEJS_DB_MAX_POOL` | The maximum number of concurrent database connections to pool. Default is `8`       | A valid number            |    ❌    |
+
+[questdb]: https://questdb.io/