Add main layout

docs/_sass/main.sass

@@ -0,0 +1,64 @@
+.bd-main
+  overflow: hidden
+  position: relative
+
+// Lead
+
+.bd-lead
+  // border: 1px solid lightskyblue
+  position: relative
+
+.bd-breadcrumb
+  margin-bottom: 2.5rem
+
+.bd-header
+  margin-bottom: 2.5rem
+  .subtitle
+    color: $text-light
+    strong
+      color: currentColor
+
+// Side
+
+.bd-side,
+.bd-side-background
+  background-color: $white-bis
+  // border: 1px solid coral
+
+.bd-side
+  position: relative
+
+.bd-side-background
+  bottom: 0
+  left: 50%
+  position: absolute
+  right: 0
+  top: 0
+
++touch
+  .bd-lead,
+  .bd-side
+    padding: 1.5rem
+
++mobile
+  .bd-side-background
+    display: none
+
++tablet
+  .bd-duo
+    display: flex
+  .bd-lead
+    background-color: $white
+    overflow: hidden
+    flex-grow: 1
+    flex-shrink: 1
+  .bd-side
+    flex: 0 0 300px
+
++desktop
+  .bd-main
+    padding: 0 3rem
+  .bd-lead
+    padding: 3rem 3rem 3rem 0
+  .bd-side
+    padding: 3rem 0 3rem 1.5rem