overview TOC: only chapter headings by default, two columns with sub-headings

The overview table of contents now lists only top-level (#) headings by
default. Adding {data-overview-subheadings=true} to the overview
heading opts in to the ## sub-headings below their chapters, which are
then laid out in two CSS columns (chapters kept intact across column
breaks).

Change-Id: Ic916c07b5479e8d19fd942e9151b0a471bf039a4
diff --git a/inst/reveal.js-5.2.1/dist/theme/ids.css b/inst/reveal.js-5.2.1/dist/theme/ids.css
index 5ae8600..16c8f5e 100644
--- a/inst/reveal.js-5.2.1/dist/theme/ids.css
+++ b/inst/reveal.js-5.2.1/dist/theme/ids.css
@@ -1074,6 +1074,13 @@
   padding: 0 0 0 1.1em;
   font-weight: normal;
 }
+.reveal .ids-overview-toc.with-subheadings {
+  columns: 2;
+  column-gap: 2.5em;
+}
+.reveal .ids-overview-toc.with-subheadings > li {
+  break-inside: avoid;
+}
 .reveal .ids-overview-toc ul li {
   margin: 0.18em 0;
 }