docs: Group arch-specific documentation under "CPU Architectures"

To declutter the top-level table of contents (the side bar), this
patch reduces the architecture-specfic documentation to one top-level
item, "CPU Architectures".

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20210312152804.2110703-1-j.neuschaefer@gmx.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>