docs: extcon: convert it to ReST and move to ACPI dir

The intel-int3496.txt file is a documentation for an ACPI driver.

There's no reason to keep it on a separate directory.

So, instead of keeping it on some random location, move it
to a sub-directory inside the ACPI documentation dir,
renaming it to .rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>