Fix markdown checkbox rendering (#17427)

We allow to render empty check list item - [ ], while GitHub doesn't allow.

To make the rendering correct, we need tune the UI (the last PR #17413 uses absolute layout, which makes the empty checkbox item can not be displayed correctly)

web_src/less/markup/content.less

@@ -159,12 +159,18 @@
   .task-list-item {
     list-style-type: none;
     position: relative;
+    line-height: 1.5rem;
+    min-height: 1.5rem; // to render a checkbox list without content `- [ ]`, we need this min-height to make sure the <li> can be visible
 
     input[type="checkbox"] {
       position: absolute;
       top: .25em;
       left: -1.6em;
     }
+
+    p {
+      line-height: 1.5rem;
+    }
   }
 
   .task-list-item + .task-list-item {