Add root doc site definition for WebRTC documentation

Bug: None
Change-Id: I64f5a356e0358360bd8326c39c8b0a898b879ebf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208641
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33360}
diff --git a/g3doc/g3doc.lua b/g3doc/g3doc.lua
new file mode 100644
index 0000000..c6ab162
--- /dev/null
+++ b/g3doc/g3doc.lua
@@ -0,0 +1,18 @@
+return {
+  theme = {
+    '@builtins/theme/ng.md'
+    -- We don't want to have more than h3 headings in the Table Of Content.
+    --toc_level = 3,
+  },
+
+  site = {
+    name = 'WebRTC C++ library',
+    home = '/g3doc/index.md',
+    logo = '/g3doc/logo.svg',
+    map  = '/g3doc/sitemap.md',
+  },
+
+  visibility = { '/...' },
+
+  freshness = {}
+}