In markdown, update summary > heading style (#1949)

* In markdown, update summary > heading style

* Update headings.scss

* Create tall-experts-hammer.md

* Update headings.scss

* Update headings.scss

* Update headings.scss

* Run stylelint

Co-authored-by: simurai <simulus@gmail.com>

Author: heiskr

.changeset/tall-experts-hammer.md

@@ -0,0 +1,5 @@
+---
+"@primer/css": patch
+---
+
+Render headings inline when in a `<summary>`

src/markdown/headings.scss

@@ -71,4 +71,21 @@
     font-size: 0.85em;
     color: var(--color-fg-muted);
   }
+
+  summary {
+    h1,
+    h2,
+    h3,
+    h4,
+    h5,
+    h6 {
+      display: inline-block;
+    }
+
+    h1,
+    h2 {
+      padding-bottom: 0;
+      border-bottom: 0;
+    }
+  }
 }