Document changes and release 5.12.2pureos3 of linux-librem5

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
2 jobs for release_5122_3 in 64 minutes and 9 seconds
Status Job ID Name Coverage
  Build
passed #352609
librem5
build:aarch64

00:54:20

passed #352610
librem5:arm64
build:deb

01:04:09