auto-populate Overview/Überblick slides with a clickable TOC

Port the convention from the old Google Docs slide workflow: at
presentation time, any slide whose (only) heading is Overview or
Überblick gets its body content replaced with a hierarchical list of
the deck's h1/h2 slide headings, each entry linking to its slide.
Styled in the ids theme; slides without headings and the title slide
are left out.

Change-Id: I3bfd62d530f297a389030e431a6066b5adc99bb8
diff --git a/man/revealjs_presentation.Rd b/man/revealjs_presentation.Rd
index 3912575..d9fefed 100644
--- a/man/revealjs_presentation.Rd
+++ b/man/revealjs_presentation.Rd
@@ -226,6 +226,14 @@
 
 The function \code{\link[=slide_timing]{slide_timing()}} computes these totals directly from an
 \code{.Rmd} source file without rendering it.
+}
+
+\subsection{Overview slides}{
+
+Like in the original Google Docs slide workflow, a slide titled
+"Overview" or "Überblick" is automatically turned into a table of
+contents: its body content is replaced at presentation time with a
+hierarchical, clickable list of all \verb{#}/\verb{##} slide headings.
 \subsection{Search}{
 
 When opt-in, it is possible to show a search box when pressing \code{CTRL + SHIFT + F}. It will seach in the whole presentation, and highlight matched words. The