docs: ipmb: place it at driver-api and convert to ReST

No new doc should be added at the main Documentation/ directory.

Instead, new docs should be added as ReST files, within the
Kernel documentation body.

Fixes: 51bd6f29 ("Add support for IPMB driver")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>