docs: Bump size of headings

Smallest heading was 1.0em which appeared same-or-smaller than the
0.9em body-text.

Increased size of all heading levels to something more subtantial,
which makes the structure of the documentation clearer

docs/ociotheme/static/ocio.css_t

@@ -245,30 +245,30 @@ div.seealso {
 
 /* More layout and styles */
 h1 {
-    font-size: 1.3em;
+    font-size: 1.7em;
     font-weight: bold;
     color: {{ theme_headingcolor }};
     border-bottom: dotted thin #e0e0e0;
     margin-top: 30px;
 }
 
 h2 {
-    font-size: 1.2em;
+    font-size: 1.6em;
     font-weight: normal;
     color: {{ theme_headingcolor }};
     border-bottom: dotted thin #e0e0e0;
     margin-top: 30px;
 }
 
 h3 {
-    font-size: 1.1em;
+    font-size: 1.4em;
     font-weight: normal;
     color: {{ theme_headingcolor }};
     margin-top: 30px;
 }
 
 h4 {
-    font-size: 1.0em;
+    font-size: 1.3em;
     font-weight: normal;
     color: {{ theme_headingcolor }};
     margin-top: 30px;