Improve webrtc documentation infra. Preview at:

https://g3doc-ng.corp.google.com/gob/webrtc/src/+/refs/changes/213189/1/g3doc/how_to_write_documentation.md

Bug: webrtc:12545
Change-Id: I284714f9e4e39f10eda03cc464ca695e8b272cd7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/213189
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33593}
diff --git a/g3doc.lua b/g3doc.lua
new file mode 100644
index 0000000..85d8474
--- /dev/null
+++ b/g3doc.lua
@@ -0,0 +1 @@
+return require(this.dirname..'g3doc/g3doc.lua')