Update SubNav docs to clarify SubNav.Heading semantic behaviour (#739)

* update SubNav docs to clarify SubNav.Heading behaviour

* hide sub-menu toggle in mobile menu

* add changeset

* update snapshots

* github-actions[bot] Regenerated snapshots

---------

Co-authored-by: joshfarrant <joshfarrant@users.noreply.github.com>

.changeset/hip-forks-taste.md

@@ -0,0 +1,5 @@
+---
+'@primer/react-brand': patch
+---
+
+Fixed a bug in the `SubNav` component where the submenu toggle was rendered as an empty button on mobile viewports.

apps/docs/content/components/SubNav.mdx

@@ -99,6 +99,8 @@ import {SubNav} from '@primer/react-brand'
 
 ### SubNav.Heading <Label>Required</Label>
 
+`SubNav.Heading` is a styled `a` tag which describes the overarching category of the other links within the SubNav. The `SubNav.Heading` is structurally identical to a `SubNav.Link` and differs only in visual styling.
+
 | name   | type     | default | required | description                      |
 | ------ | -------- | ------- | -------- | -------------------------------- |
 | `href` | `string` |         |          | URL of the page the link goes to |

packages/react/src/SubNav/SubNav.module.css

@@ -52,6 +52,10 @@
   padding: 0;
 }
 
+.SubNav__sub-menu-toggle {
+  display: none;
+}
+
 /*
  * Narrow breakpoint
  */

packages/react/src/SubNav/SubNav.module.css.d.ts

@@ -8,6 +8,7 @@ declare const styles: {
   readonly "SubNav__sub-menu-icon": string;
   readonly "SubNav__sub-menu-children": string;
   readonly "SubNav__links-overlay": string;
+  readonly "SubNav__sub-menu-toggle": string;
   readonly "SubNav--open": string;
   readonly "fade-in": string;
   readonly "SubNav__links-overlay--open": string;
@@ -19,7 +20,6 @@ declare const styles: {
   readonly "SubNav__overlay-toggle-content": string;
   readonly "SubNav__link-label": string;
   readonly "SubNav__link--expanded": string;
-  readonly "SubNav__sub-menu-toggle": string;
   readonly "SubNav__link--has-sub-menu": string;
   readonly "fade-in-down": string;
 };