docs: i2c: sort index logically

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 <luca@lucaceresoli.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>