docs: powerpc: Add htm.rst to table of contents

docs: powerpc: Add htm.rst to table of contents

Fix the following documentation build error, which was introduced when
Documentation/arch/powerpc/htm.rst was added to the repository without
any reference to the document.

Documentation/arch/powerpc/htm.rst: WARNING: document isn't included in any toctree [toc.not_included]

Fixes: ab1456c5aa7a ("powerpc/pseries/htmdump: Add documentation for H_HTM debugfs interface")
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Brigham Campbell <me@brighamcampbell.com>
Tested-by: Athira Rajeev <atrajeev@linux.ibm.com>
Fixes: ab1456c5aa7a63d5 ("powerpc/pseries/htmdump: Add documentation for H_HTM debugfs interface")
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250528054146.2658537-2-me@brighamcampbell.com