Documentation: extcon: add documentation for Extcon subsystem

The Extcon (External Connector) subsystem driver lacked proper
documentation. This commit adds comprehensive documentation
explaining the purpose, key components, and usage of the Extcon
framework.

The new documentation includes:
- An overview of the Extcon subsystem
- Descriptions of key structures
- Explanations of core functions
- Information on the sysfs interface
- A usage example for driver developers

Link: https://lore.kernel.org/lkml/20241103025436.69196-1-yesanishhere@gmail.com/


Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: anish kumar <yesanishhere@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>