doc: modbus: remove extra level in the docs

We had one extra level in the docs, reduce and show modbus details
directly instead of the extra step.

Signed-off-by: Anas Nashif <[email protected]>

Author: nashif

doc/reference/modbus/index.rst

@@ -3,7 +3,41 @@
 MODBUS
 ######
 
-.. toctree::
-   :maxdepth: 2
+Modbus is an industrial messaging protocol. The protocol is specified
+for different types of networks or buses. Zephyr OS implementation
+supports communication over serial line and may be used
+with different physical interfaces, like RS485 or RS232.
+TCP support is not implemented directly, but there are helper functions
+to realize TCP support according to the application's needs.
 
-   modbus.rst
+Modbus communication is based on client/server model.
+Only one client may be present on the bus. Client can communicate with several
+server devices. Server devices themselves are passive and must not send
+requests or unsolicited responses.
+Services requested by the client are specified by function codes (FCxx),
+and can be found in the specification or documentation of the API below.
+
+Zephyr RTOS implementation supports both client and server roles.
+
+More information about Modbus and Modbus RTU can be found on the website
+`MODBUS Protocol Specifications`_.
+
+Samples
+*******
+
+`modbus-rtu-server-sample` and `modbus-rtu-client-sample` give
+the possibility to try out RTU server and RTU client implementation with
+an evaluation board.
+
+`modbus-tcp-server-sample` is a simple Modbus TCP server.
+
+`modbus-gateway-sample` is an example how to build a TCP to serial line
+gateway with Zephyr OS.
+
+API Reference
+*************
+
+.. doxygengroup:: modbus
+   :project: Zephyr
+
+.. _`MODBUS Protocol Specifications`: https://www.modbus.org/specs.php