update and move old IO documentation to legacy page

doc/PodioInputOutput.md → doc/LegacyPodioInputOutput.md

@@ -16,14 +16,18 @@ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
 -->
-# Reading and writing EDM4hep files in Gaudi
-
-The facilities to read and write EDM4hep (or in general event data models based
-on podio) are provided by [`k4FWCore`](https://github.com/key4hep/k4FWCore).
-This page will describe their usage, but not go into too much details of their
-internals. This page also assumes a certain familiarity with Gaudi, i.e. most of
-the snippets just show a minimal configuration part, and not a complete runnable
-example.
+# Legacy reading and writing EDM4hep files in Gaudi with the legacy k4DataSvc
+
+:::{caution}
+`k4DataSvc` is a legacy service previously used in K4FWCore for reading and writing data in EDM4hep or other data models based on PODIO.
+
+The currently used service is `IOSvc`, which offers improved streamlined functionality and better support for modern workflows. For detailed documentation on `IOSvc`, refer to [this documentation](IO.md).
+:::
+
+This page will describe the usage of legacy [k4FWCore](https://github.com/key4hep/k4FWCore)
+facilities to read and write EDM4hep. This page also assumes a certain
+familiarity with Gaudi, i.e. most of the snippets just show a minimal
+configuration part, and not a complete runnable example.
 
 ## The `k4DataSvc`