Add CESSDA SKG-IF API in Tools (#32)

* Add CESSDA SKG-IF API in Tools

* Issues fixed so edit status accordingly

* Fix formatting of SKG tools list in documentation

Author: markusjt

docs/tools/SKG.rst

@@ -11,3 +11,4 @@ The following resources in this section are part of the FAIR Tools component.
     OpenAIRE Graph SKG-IF API <SKG_tools/openaire>
     RO-HUB SKG-IF API <SKG_tools/rohub>
     LifeWatch Eric KG <SKG_tools/lifewatch>
+    CESSDA SKG-IF API <SKG_tools/cessda>

docs/tools/SKG_tools/cessda.rst

@@ -0,0 +1,25 @@
+
+CESSDA SKG-IF API
+=================
+
+Overview
+--------
+
+For links to OpenAPI documentation, implemented endpoints and examples of filter usage:
+
+`https://skg-if-staging.cessda.eu/ <https://skg-if-staging.cessda.eu/>`__
+
+Endpoints
+---------
+
+* `https://skg-if-staging.cessda.eu/products <https://skg-if-staging.cessda.eu/products>`__ – Returns studies from the CESSDA Data Catalogue (CDC).
+* `https://skg-if-staging.cessda.eu/topics <https://skg-if-staging.cessda.eu/topics>`__ – Returns topics from the European Language Social Science Thesaurus (ELSST).
+
+Current Status
+--------------
+
+Version 0.1.0 has been released with minor fixes since then. The Products endpoint may respond slowly but should return all CDC studies without any errors.
+
+.. note::
+
+    This is a staging environment intended for testing. Availability is not guaranteed, and services may be down without prior notice. Endpoints, data, and behavior are subject to change at any time.