docs: i2c: sort index logically

The index page currently lists sections in alphabetical file order without
caring about their content. Sort sections based on their content logically,
according to the following structure:

* Intro to I2C/SMBus and their usage in Linux: summary, i2c-protocol,
smbus-protocol, instantiating-devices, busses/index, i2c-topology,
muxes/i2c-mux-gpio
* Implementing drivers: writing-clients, dev-interface,
dma-considerations, fault-codes, functionality
* Debugging: gpio-fault-injection, i2c-stub
* Slave I2C: slave-interface, slave-eeprom-backend
* Advanced: ten-bit-addresses
* Obsolete info: upgrading-clients, old-module-parameters

Signed-off-by: Luca Ceresoli <[e-mail]>
Reviewed-by: Jean Delvare <[e-mail]>
Signed-off-by: Wolfram Sang <[e-mail]>

Documentation/i2c/index.rst

···
 I2C/SMBus Subsystem
 ===================
 
+Introduction
+============
+
 .. toctree::
    :maxdepth: 1
 
+   summary
+   i2c-protocol
+   smbus-protocol
+   instantiating-devices
+   busses/index
+   i2c-topology
+   muxes/i2c-mux-gpio
+
+Writing device drivers
+======================
+
+.. toctree::
+   :maxdepth: 1
+
+   writing-clients
    dev-interface
    dma-considerations
    fault-codes
    functionality
+
+Debugging
+=========
+
+.. toctree::
+   :maxdepth: 1
+
    gpio-fault-injection
-   i2c-protocol
    i2c-stub
-   i2c-topology
-   instantiating-devices
-   old-module-parameters
-   slave-eeprom-backend
+
+Slave I2C
+=========
+
+.. toctree::
+   :maxdepth: 1
+
    slave-interface
-   smbus-protocol
-   summary
+   slave-eeprom-backend
+
+Advanced topics
+===============
+
+.. toctree::
+   :maxdepth: 1
+
    ten-bit-addresses
+
+Legacy documentation
+====================
+
+.. toctree::
+   :maxdepth: 1
+
    upgrading-clients
-   writing-clients
-
-   muxes/i2c-mux-gpio
-
-   busses/index
+   old-module-parameters
 
 .. only::  subproject and html