Add graphql-api docs section

docs/guides/developer/docs/architecture/graphql/index.md

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+# GraphQL API - Technology Preview
+
+## Introduction
+
+The Opencast GraphQL API is a modern, flexible, and powerful alternative to the existing REST API. It is designed to provide a more efficient and flexible way to interact with Opencast. The GraphQL API is available as Tech Preview it is not yet ready for production and may change without notice.
+
+## Architectural Overview
+
+The GraphQL API is a single endpoint that allows clients to query and mutate data. The API is based on the [GraphQL](https://graphql.org/) query language. The API is designed to be flexible and efficient, allowing clients to request only the data they need in a single request.
+
+### Url Space
+The GraphQL API is located at the `/graphql` namespace on the Opencast admin node. This results in all requests to the
+External API starting with `https://<hostname>/graphql`, where the hostname is depending on the installation and tenant
+(see “Multi Tenancy”). The whole schema is available at `https://<hostname>/graphql/schema.json`.
+
+### GraphQL UI
+The Opencast-Admin node comes with a built-in UI that allows you to explore the schema and execute queries. The UI is available at `https://<hostname>/graphql-ui`.