docs: powerpc: convert docs to ReST and rename to *.rst

Convert docs to ReST and add them to the arch-specific
book.

The conversion here was trivial, as almost every file there
was already using an elegant format close to ReST standard.

The changes were mostly to mark literal blocks and add a few
missing section title identifiers.

One note with regards to "--": on Sphinx, this can't be used
to identify a list, as it will format it badly. This can be
used, however, to identify a long hyphen - and "---" is an
even longer one.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com> # cxl