docs: powerpc: add htm.rst to toctree

The file Documentation/arch/powerpc/htm.rst is not included in the
index.rst toctree. This results in a warning when building the docs:

  WARNING: document isn't included in any toctree: htm.rst

Add it to the index.rst file so that it is properly included in the
PowerPC documentation TOC.

Signed-off-by: Vishal Parmar <vishistriker@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250727110145.839906-1-vishistriker@gmail.com