docs: Adjust heading sizes (#33628)

Just fine-tuning some heading sizes that were off, particularly h4s and
h5s.

Release Notes:

- N/A

docs/theme/css/general.css

@@ -79,20 +79,34 @@ h6 code {
   display: none !important;
 }
 
+h1 {
+  font-size: 3.4rem;
+}
+
 h2 {
   padding-bottom: 1rem;
   border-bottom: 1px solid;
   border-color: var(--border-light);
 }
 
-h2,
 h3 {
-  margin-block-start: 1.5em;
-  margin-block-end: 0;
+  font-size: 2rem;
 }
+
+h4 {
+  font-size: 1.8rem;
+}
+
+h5 {
+  font-size: 1.6rem;
+}
+
+h2,
+h3,
 h4,
 h5 {
-  margin-block-start: 2em;
+  margin-block-start: 1.5em;
+  margin-block-end: 0;
 }
 
 .header + .header h3,