Move threading documentation for API into g3doc structure

Bug: webrtc:12674
Change-Id: I49bb46b4e505f89ce8d56c469a8995779edf1f28
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/214969
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33690}

api/g3doc/index.md

@@ -0,0 +1,13 @@
+<?% config.freshness.owner = 'hta' %?>
+<?% config.freshness.reviewed = '2021-04-12' %?>
+
+# The WebRTC API
+
+The public API of the WebRTC library consists of the api/ directory and
+its subdirectories. No other files should be depended on by webrtc users.
+
+Before starting to code against the API, it is important to understand
+some basic concepts, such as:
+
+* Memory management, including webrtc's reference counted objects
+* [Thread management](threading_design.md)

api/g3doc/threading_design.md

@@ -1,4 +1,6 @@
-# Design considerations
+<?% config.freshness.owner = 'hta' %?>
+<?% config.freshness.reviewed = '2021-04-12' %?>
+# API Threading Design considerations
 
 The header files in this directory form the API to the WebRTC library
 that is intended for client applications' use.
@@ -30,7 +32,7 @@ the two calls.
 sequential execution - other names for such constructs are task runners
 and sequenced task queues.
 
-# Client threads and callbacks
+## Client threads and callbacks
 
 At the moment, the API does not give any guarantee on which thread* the
 callbacks and events are called on. So it's best to write all callback
@@ -47,7 +49,7 @@ void ObserverClass::Handler(event) {
 In the future, the implementation may change to always call the callbacks
 and event handlers on the client thread.
 
-# Implementation considerations
+## Implementation considerations
 
 The C++ classes that are part of the public API are also used to derive
 classes that form part of the implementation.

g3doc/sitemap.md

@@ -2,7 +2,8 @@
 *   How to contribute
     *   Code
     *   [Documentation](/g3doc/how_to_write_documentation.md)
-*   Public C++ API
+*  [Public C++ API](/api/g3doc/index.md)
+    *   [Threading](/api/g3doc/threading_design.md) 
 *   Implementation
     *   Network
     *   Congestion control and bandwidth estimation