Add coupling documentation. (#1481)

Forgot to add this in #1137, currently the website can't find this page.

docs/core/coupling.qmd

@@ -0,0 +1,24 @@
+---
+title: "Coupling"
+---
+
+# Coupling
+
+## iMOD
+
+Ribasim can also be (online) coupled to other kernels with the help of iMOD Coupler. The corresponding documentation can be found within the [iMOD Suite Documentation](https://deltares.github.io/iMOD-Documentation/coupler.html).
+
+## Water quality
+
+Ribasim can be offline coupled to Delwaq, the Deltares Water Quality model. Note that this functionality is still in active development.
+
+```{mermaid}
+flowchart LR
+    Ribasim --> Delwaq
+```
+
+### Setup
+
+Delwaq can calculate the concentration of substances in Ribasim Basin nodes over time, based on initial concentrations in basins, and of boundary flows. Ribasim exposes the [Basin / concentration](usage.qmd#sec-basin-conc), [Basin / concentrationstate](usage.qmd#sec-basin-conc-state), [FlowBoundary / concentration](usage.qmd#sec-flow-boundary-conc), and [LevelBoundary / concentration](usage.qmd#sec-level-boundary-conc) tables to setup these substances and concentrations.
+
+When a Ribasim model has been run with the above tables, one can use the utilities in the `coupling/delwaq` folder to generate the input required for Delwaq to run, as well as to parse the output from Delwaq into a Ribasim compatible format. For more information see the `README.md` in the same folder.