DOCS: Add tutorial for writing converter functions

docs/user_guide/examples/explanation_grids.ipynb

@@ -12,7 +12,7 @@
     "Parcels can natively handle two styles of grids; structured and unstructured, where Parcels `Field` objects exist on a (structured) {py:obj}`parcels.XGrid` and conform to [SGRID](https://sgrid.github.io/sgrid/) conventions, or on a (unstructured) {py:obj}`parcels.UxGrid` and conform to [UGRID](https://ugrid-conventions.github.io/ugrid-conventions/) conventions. Here we describe these grids on a conceptual level.\n",
     "\n",
     "```{note}\n",
-    "Parcels comes ready with a range of `convert` functions which will convert your `xarray` dataset into a SGRID or UGRID compliant dataset. See [this list](../index.md#converting-model-data-to-fieldsets) for all the models we have `convert` functions for. If your model is not on that list, you can create your own `convert` function, or reach out on our discussion board for help.\n",
+    "Parcels comes ready with a range of `convert` functions which will convert your `xarray` dataset into a SGRID or UGRID compliant dataset. See [this list](../index.md#converting-model-data-to-fieldsets) for all the models we have `convert` functions for. If your model is not on that list, you can [create your own `convert` function](./explanation_writing_convert_functions.md), or reach out on our discussion board for help.\n",
     "```\n",
     "\n",
     "Under the hood, every `Field` in a `FieldSet` has a `grid` attribute. This `grid` stores the spatial and temporal information of the Field coordinates. The number of Grids in a FieldSet is thus always smaller or equal to the number of Field objects; and this is what the \"grid number\" column in {py:func}`parcels.FieldSet.describe()` refers to.\n",