proc*.5: Make sashimi

[Merge tag 'proc-sashimi-v1' of <git://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git>]

proc(5) was a huge page, which was quite hard to maintain, extend, read,
and refer to.  Split the page into small pages for the different
directories and files within /proc.  Some pages are still too large
(e.g., proc_sys(5)), and will some day be split even more.

This split keeps the contents of the original page, without modifying
anything; not even the formatting.

The only thing that has been modified in this patches, is that
directories are consistently represented with a trailing slash.

For the file name of the pages, we've used the name of the interface
(e.g., /proc/pid/), removing the leading and trailing '/'s and then
translating the remaining ones as `tr / _` (e.g., proc_pid.5).  The
title of the pages (TH) is consistent with this.  The NAME of the pages,
however, is the actual path name of the interfaces.

The man page references have not been updated, as that was a more
complex and tedious work, so I expect that they'll be slowly updated as
we and users find out.

Link: <https://lore.kernel.org/linux-man/e3a5bc09-e835-9819-4aaa-12959495ac59@kernel.org/T/>
Acked-by: Oskari Pirhonen <xxc3ncoredxx@gmail.com>
Acked-by: Günther Noack <gnoack@google.com>
Acked-by: "G. Branden Robinson" <g.branden.robinson@gmail.com>
Cc: Brian Inglis <Brian.Inglis@Shaw.ca>
Cc: Ingo Schwarze <schwarze@usta.de>
Cc: Colin Watson <cjwatson@debian.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>