Create rest-api.md - an overview of all REST API versions

Publisher/en/rest-api.md

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+# REST API Overview
+
+The Copernica REST API has multiple versions. This page contains information 
+and links for all versions, along with some details about changes in each version.
+
+The newest version of the API is version 4, which is the recommended version
+to use. However, all of the previous versions are still operational. Here is
+a list of all REST API versions:
+
+*  [REST API v4](./restv4/rest-api.md)
+*  [REST API v3](./restv3/rest-api.md)
+*  [REST API v2](./restv2/rest-api.md)
+*  [REST API v1](./restv1/rest-api.md)
+
+
+## REST API v4
+
+Version 4 of the API uses JWT authorization and contains additional endpoints.
+
+More details about this version can be found [here](./restv4/rest-introduction.md). 
+
+### REST API v3
+
+Version 3 of the API contains additional endpoints and some improvments over
+the previous version.
+
+More details about this version can be found [here](./restv3/rest-introduction.md). 
+
+### REST API v2
+
+Version 2 of the REST API contains improvments, extra endpoints and 
+separate sections for Publisher and Marketing Suite.
+
+More details about this version can be found [here](./restv2/rest-introduction.md). 
+
+### REST API v1
+
+The initial, version 1, of the Copernica REST API. It is considered outdated,
+but is still operational. 
+
+More details about this version can be found [here](./restv1/rest-introduction.md).