Add GraphQL documentation

Author: Julien Neuhart

docs/docs/06_GraphQL/1_Overview.md

@@ -1,6 +1,23 @@
 ---
 title: Overview
-slug: /lists
+slug: /graphql
 ---
 
-🚧
+> GraphQL is a query language for APIs and a runtime for fulfilling those queries with your existing data. 
+> GraphQL provides a complete and understandable description of the data in your API, gives clients the power to ask 
+> for exactly what they need and nothing more, makes it easier to evolve APIs over time, and enables powerful 
+> developer tools.
+>
+> -- <cite>[graphql.org](https://graphql.org/)</cite>
+
+The Symfony Boilerplate uses [GraphQLite](https://graphqlite.thecodingmachine.io/) to quickly build a GraphQL API based
+on your models and use cases.
+
+As the documentation of this PHP library covers a lot of aspects, we invite you to read it to have a better understanding
+of its functionalities.
+
+## Tools
+
+The API provides the endpoint `/graphiql` (in development) so that you may quickly test a query or a mutation.
+
+You may also use the [Altair GraphQL Client](https://altair.sirmuel.design/).