Update GraphQL guide

- removed the GraphQL feature flag mention
- added information about GraphiQL

doc/development/fe_guide/graphql.md

 # GraphQL
 
+Our GraphQL API can be explored via GraphiQL at your instance's
+`/-/graphql-explorer` or at [GitLab.com](https://gitlab.com/-/graphql-explorer).
+
+You can check all existing queries and mutations on the right side
+of GraphiQL in its **Documentation explorer**. It's also possible to
+write queries and mutations directly on the left tab and check
+their execution by clicking **Execute query** button on the top left:
+
+![GraphiQL interface](img/graphiql_explorer_v12_4.png)
+
 We use [Apollo] and [Vue Apollo][vue-apollo] for working with GraphQL
 on the frontend.
 
-In order to use GraphQL, you need to enable the `graphql` feature flag,
-read more about [Feature Flags][feature-flags].
-
 ## Apollo Client
 
 To save duplicated clients getting created in different apps, we have a