User-guide update

.gitlab-ci.yml

.gitlab-ci/packaging.yml

CHANGELOG.md

docker/nektar-env/buster_default_packages.txt

docker/nektar-env/buster_full_packages.txt

docker/packaging/Dockerfile-debian-buster

docs/user-guide/installation/precompiled.tex

library/LibUtilities/Communication/CommMpi.cpp

pkg/debian/buster/compat

pkg/debian/buster/control

pkg/debian/buster/copyright

pkg/debian/buster/gbp.conf

pkg/debian/buster/libnektar++-dev.install

pkg/debian/buster/libnektar++-doc.install

pkg/debian/buster/libnektar++-doc.lintian-overrides

pkg/debian/buster/libnektar++.install

pkg/debian/buster/libnektar++.lintian-overrides

pkg/debian/buster/nekmesh.install

pkg/debian/buster/nekmesh.manpages

pkg/debian/buster/nektar++-adr-solver.install