add a collapsible ToC

assets/styles/base.scss

@@ -23,6 +23,23 @@ p, ul, text {
   padding: revert;
 }
 
+.mainTOC {
+  background: var(--lightgray);
+  border-radius: 8px;
+  padding: 0.8em;
+}
+
+.mainTOC details summary {
+  cursor: zoom-in;
+  font-family: Inter;
+  color: var(--dark);
+  font-weight: 700;
+}
+
+.mainTOC details[open] summary {
+    cursor: zoom-out;
+}
+
 #TableOfContents > ol {
   counter-reset: section;
   margin-left: 0em;