Add documentation about reading query plans

This adds a database guide on how to read the output of "EXPLAIN" and
"EXPLAIN ANALYZE", and how to use this output to understand a query's
performance and optimise it.

doc/development/README.md

@@ -55,7 +55,13 @@ description: 'Learn how to contribute to GitLab.'
 - [Merge request performance guidelines](merge_request_performance_guidelines.md)
   for ensuring merge requests do not negatively impact GitLab performance
 
-## Databases guides
+## Database guides
+
+### Tooling
+
+- [Understanding EXPLAIN plans](understanding_explain_plans.md)
+- [explain.depesz.com](https://explain.depesz.com/) for visualising the output
+  of `EXPLAIN`
 
 ### Migrations