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 <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Signed-off-by: Brigham Campbell <[e-mail]>
Tested-by: Athira Rajeev <[e-mail]>
Fixes: ab1456c5aa7a63d5 ("powerpc/pseries/htmdump: Add documentation for H_HTM debugfs interface")
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/arch/powerpc/index.rst

···
     elf_hwcaps
     elfnote
     firmware-assisted-dump
+    htm
     hvcs
     imc
     isa-versions