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 <[email protected]>
Signed-off-by: Madhavan Srinivasan <[email protected]>
Link: https://patch.msgid.link/[email protected]

Author: Vishalsinh-Parmar

Documentation/arch/powerpc/index.rst

@@ -36,6 +36,7 @@ powerpc
     vas-api
     vcpudispatch_stats
     vmemmap_dedup
+    htm
 
     features