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: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/driver-api/index.rst

@@ -76,6 +76,7 @@ available subsections can be seen below.
    dell_rbu
    edid
    eisa
+   ipmb
    isa
    isapnp
    generic-counter