fix css for inheritance diagramsize

Author: songmeo

docs/static/custom.css

@@ -1,3 +1,12 @@
+/* See https://github.com/OpenCyphal/pycyphal/issues/321 */
+object.inheritance, img.inheritance {
+    max-width: 25%;
+    width: 25%;
+    height: auto;
+    display: block;
+    margin: auto;
+}
+
 /* Gray text is ugly. Text should be black. */
 body {
     color: #000;