Add video/g3doc/stats.md to the doc site menu

Bug: webrtc:12545, webrtc:12563
Change-Id: Id5db7148030e5d7d952dad4d7a30993ac2f72db5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215400
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33758}
diff --git a/g3doc/sitemap.md b/g3doc/sitemap.md
index 43e1440..f3f9cd7 100644
--- a/g3doc/sitemap.md
+++ b/g3doc/sitemap.md
@@ -23,6 +23,7 @@
     *   Video
         *   [Adaptation](/video/g3doc/adaptation.md)
         *   [Video coding](/modules/video_coding/g3doc/index.md)
+        *   [Stats](/video/g3doc/stats.md)
     *   DataChannel
     *   PeerConnection
     *   Desktop capture