docs: add custom CSS file (copied from khiops-python)

Author: popescu-v

docs/_static/css/custom.css

@@ -0,0 +1,24 @@
+/* Custom header sizes */
+h1 {
+  font-size: 1.85em;
+}
+
+h2 {
+  font-size: 1.55em;
+}
+
+h3 {
+  font-size: 1.30em;
+}
+
+h4 {
+  font-size: 1.10em;
+}
+
+h5 {
+  font-size: 0.90em;
+}
+
+img.sidebar-logo {
+  width: 40px;
+}