rtd-ci: add read-the-docs integration

documentation/_static/css/custom.css

@@ -0,0 +1,7 @@
+.wy-side-nav-search {
+    background-color: #18334B;
+}
+
+.wy-side-nav-search input[type="text"] {
+    border-color: #18334b;
+}