website/docs: Apple Business Manager Integration. (#13319)

website/docs: Flesh out Apple Business Manager integration.

- Fix missing required attribute.
- Adjust visible headers in table of contents.
- Fix Docusaurus warning for nested paragraphs.

website/src/css/custom.css

@@ -207,7 +207,7 @@ body {
     /* Improve contrast. */
     & .messageText {
         stroke: var(--ifm-background-color) !important;
-        stroke-width: 10;
+        stroke-width: 4;
         fill: var(--ifm-color-content) !important;
         paint-order: stroke;
     }
@@ -220,3 +220,14 @@ body {
         display: none;
     }
 }
+
+.contains-task-list {
+    input[type="checkbox"] {
+        appearance: none;
+    }
+    input[type="checkbox"]::after {
+        content: "✓";
+        color: var(--ifm-color-success-dark);
+        display: inline-block;
+    }
+}