Attached patch adds buffer-local registers to doc-view-mode.  I have
opted to make the registers buffer-local since I personally use the same
mnemonic to save pages to the register in multiple buffers.  It is
something like folding a page or placing a pen in between to quickly go
to-and-fro between those pages.

In GNU Emacs 31.0.50 (build 7, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.18.0, Xaw scroll bars) of 2024-09-09 built on astatine
Repository revision: 7d7aa65f63db78c5732f1580213fc3767b767a4a
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101011
System Description: Debian GNU/Linux trixie/sid

Configured using:
 'configure --with-sound=alsa --with-x-toolkit=lucid --without-xaw3d
 --without-gconf --without-libsystemd --with-cairo'