Add open-graph tags support

Author: Jonas Chapuis

.idea/.gitignore

@@ -42,7 +42,14 @@ lazy val root = project("paradox-material-theme-parent", file("."))
     }
     // #social
     ,
-    // #language
+    // #open-graph
+    Compile / paradoxProperties ++= Map(
+      "project.description" -> "Paradox Material Theme is a theme for Paradox, a static site generator geared towards project documentation",
+      "project.image" -> "https://jonas.github.io/paradox-material-theme/images/material.png"
+    )
+    //#open-graph
+    ,
+    //#language
     Compile / paradoxMaterialTheme ~= {
       _.withLanguage(java.util.Locale.ENGLISH)
     }

.idea/misc.xml

@@ -225,6 +225,12 @@ on the URL:
 
   [17]: http://fontawesome.io/icons/
 
+## Open graph metadata
+To enable a nice preview of the site in social media, define the
+following properties in the build:
+
+@@ snip [build.sbt]($root$/build.sbt) { #open-graph }
+
 ## Copyright Notice
 
 To display a copyright notice in the footer configure the