docs: powerpc: Add missing documentation reference

docs: powerpc: Add missing documentation reference

The documentation pages for 'elfnote' and 'ultravisor'
are not included in the powerpc documentation index, this
generates Sphinx warnings:

WARNING: document isn't included in any toctree

Additionally, when one includes these missing doc pages,
more Sphinx warnings appear. Unused footnote references,
syntax highlighting and table of content ordering has
been adjusted.

Signed-off-by: Adam Zerella <adam.zerella@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190915052905.13431-1-adam.zerella@gmail.com