[docs] added link for OpenThread China and updated Doxygen TOC (#3261)

README.md

@@ -49,6 +49,8 @@ All end-user documentation and guides are located at [openthread.io](https://ope
 
 ...then [openthread.io](https://openthread.io) is the place for you.
 
+> Note: For users in China, end-user documentation is available at [openthread.google.cn](https://openthread.google.cn).
+
 If you're interested in contributing to OpenThread, read on.
 
 # Contributing

include/openthread/error.h

@@ -93,6 +93,7 @@ extern "C" {
  *
  * @{
  *
+ * @defgroup api-border-agent   Border Agent
  * @defgroup api-border-router  Border Router
  * @defgroup api-commissioner   Commissioner
  * @defgroup api-thread-general General
@@ -124,6 +125,7 @@ extern "C" {
  * @defgroup api-crypto              Crypto
  * @defgroup api-factory-diagnostics Factory Diagnostics
  * @defgroup api-jam-detection       Jam Detection
+ * @defgroup api-logging             Logging
  * @defgroup api-ncp                 Network Co-Processor
  * @defgroup api-network-time        Network Time Synchronization
  *