chore: add doxygen.extra.css

Author: David Arutiunian

doxygen.extra.css

@@ -0,0 +1,23 @@
+html,
+body {
+    height: 100%;
+}
+
+body {
+    display: flex;
+    flex-direction: column;
+}
+
+.PageDoc,
+.contents {
+    flex: 1;
+}
+
+.PageDoc {
+    /* Github like style */
+    margin: 16px auto;
+    width: 1012px;
+    padding: 32px;
+    border: 1px solid #e1e4e8;
+    border-radius: 2px;
+}