Rewrite migration guide (#33834)

- De-duplicates bulk of the content for a migration from v4 to v5
- Adds clear breaking labels where appropriate
- Leaves room for more guidance around doing the actual migration

Author: mdo

site/assets/scss/_content.scss

@@ -21,6 +21,13 @@
   > ul li,
   > ol li {
     margin-bottom: .25rem;
+
+    // stylelint-disable selector-max-type
+    > ul {
+      margin-top: -.5rem;
+      margin-bottom: 1rem;
+    }
+    // stylelint-enable selector-max-type
   }
 
   // Override Bootstrap defaults