thermal: Move ABI documentation to Documentation/ABI

The thermal ABI and the internal development details are described in:

	Documentation/driver-api/thermal/sysfs-api.rst

Move the sysfs ABI description to Documentation/ABI, ensuring that
scripts/get_abi.pl will properly parse it.

While here, also update the thermal record in MAINTAINERS to point to
the documentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
[ rjw: Changelog end subject edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>