Add link to OpenPMD tool

Author: TomGoffrey

content/documentation/visualising_output/OpenPMD_Conversion.md

@@ -0,0 +1,18 @@
+---
+draft: false
+toc: false
+type: docs
+
+title: OpenPMD Conversion
+linktitle: OpenPMD Conversion
+weight: 340
+menu:
+  documentation:
+    parent: Visualising output
+    weight: 40
+---
+
+# OpenPMD Conversion
+
+An [externally developed tool](https://github.com/denisbertini/mpi-sdf2opmd) to
+convert SDF files to the [OpenPMD](https://www.openpmd.org) format is available.