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
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 70ae148..3fe6170 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -143,6 +143,7 @@
    arm64/index
    ia64/index
    m68k/index
+   powerpc/index
    riscv/index
    s390/index
    sh/index