[web] Added process coupling feature.

web/content/_index.pandoc

@@ -80,6 +80,12 @@ features:
       url: "/docs/tools"
   layout: left
   anchor: tools
+- headline: Process Coupling
+  textline: |
+    A coupled system of equations can be either solved in a fully coupled way of the monolithic method, or in the sequential manner of the staggered scheme. The monolithic scheme is applied for all coupled processes, while the staggered scheme are available for the coupled processes of themo-hydraulic, hydro-mechanical, and phase field mechanical problems.
+  visual:
+    permalink: /features/StaggeredCouplingScheme.png
+    alt: Staggered coupling scheme
 - headline: Transparent development workflows
   textline: |
     Write front-end code with Jekyll or Hugo and push to your Git repo. Forestry will pull in your commits and update the CMS.