Add docs for main menu block

docs/building_blocks_for_flexible_content.md

@@ -147,9 +147,33 @@ A simple image.
       tablet_2x: "landing_page/placeholder/tablet_2x.png"
 ```
 
-#### Main Navigation
+#### Main Menu
 
-The landing-page top-level navigation. It supports a row of links, and sub-menus that will only appear on mobile (where the entire menu is compressed into a single drop-down). The only supported keys are the `type` key and the `navigation_group_id` key, which should point to an id of a [navigation group](#navigation-groups) to use for the menu items. Note that the styling of the first link is different, and it is assumed to be the homepage in a group of landing pages.
+The landing-page top-level navigation. It supports a heading with a row of links.
+
+##### Example
+
+```yaml
+- type: main_menu
+  button_label: "Landing page menu"
+  groups:
+  - heading: "First heading"
+    links:
+      - text: Landing page homepage very long text here to demonstrate wrapping
+        href: "/landing-page/homepage"
+      - text: Landing page homepage
+        href: "/landing-page/homepage"
+      - text: Landing page homepage
+        href: "/landing-page/homepage"
+  - heading: "Goals"
+    links:
+      - text: Goal 1
+        href: "/landing-page/goal-1"
+```
+
+#### Main Navigation (Legacy)
+
+The legacy landing-page top-level navigation. It supports a row of links, and sub-menus that will only appear on mobile (where the entire menu is compressed into a single drop-down). The only supported keys are the `type` key and the `navigation_group_id` key, which should point to an id of a [navigation group](#navigation-groups) to use for the menu items. Note that the styling of the first link is different, and it is assumed to be the homepage in a group of landing pages.
 
 ##### Example