docs: move IPMI.txt to the driver API book

The IPMI is under drivers/char. This doc describes the kAPI
part of the IPMI (mainly).

So, move it to the driver-api directory and add it to the
corresponding index.rst file.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/ae4edbc6753dc522cb93de36800978e3b58e0cfb.1588345503.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/index.rst

@@ -39,6 +39,7 @@ available subsections can be seen below.
    spi
    i2c
    ipmb
+   ipmi
    i3c/index
    interconnect
    devfreq