Updates and cleanup for Arch, RHEL, NixOS and Fedora

Signed-off-by: Maurice Zhou <ja@apvc.uk>

docs/Getting Started/RHEL-based distro/RHEL-based distro Root on ZFS/1-preparation.rst

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+.. highlight:: sh
+
+Preparation
+======================
+
+.. contents:: Table of Contents
+   :local:
+
+#. Disable Secure Boot. ZFS modules can not be loaded if Secure Boot is enabled.
+#. Download a variant of `AlmaLinux Minimal Live ISO
+   <https://repo.almalinux.org/almalinux/9/live/x86_64/>`__ and boot from it.
+#. Connect to the Internet.
+#. Set root password or ``/root/.ssh/authorized_keys``.
+#. Start SSH server::
+
+    echo PermitRootLogin yes >> /etc/ssh/sshd_config
+    systemctl restart sshd
+
+#. Connect from another computer::
+
+    ssh root@192.168.1.19
+
+#. Target disk
+
+   List available disks with::
+
+    ls /dev/disk/by-id/*
+
+   If using virtio as disk bus, use ``/dev/disk/by-path/*``.
+
+   Declare disk array::
+
+    DISK='/dev/disk/by-id/ata-FOO /dev/disk/by-id/nvme-BAR'
+
+   For single disk installation, use::
+
+    DISK='/dev/disk/by-id/disk1'
+
+#. Set partition size:
+
+   Set swap size. It's `recommended <https://chrisdown.name/2018/01/02/in-defence-of-swap.html>`__
+   to setup a swap partition. If you intend to use hibernation,
+   the minimum should be no less than RAM size. Skip if swap is not needed::
+
+    INST_PARTSIZE_SWAP=8
+
+   Root pool size, use all remaining disk space if not set::
+
+    INST_PARTSIZE_RPOOL=
+
+#. Temporarily set SELinux to permissive in live environment::
+
+    setenforce 0
+
+   SELinux will be enabled on the installed system.
+
+#. Add ZFS repo::
+
+    dnf install -y https://zfsonlinux.org/epel/zfs-release-el-2-1.noarch.rpm
+
+#. Check available repos::
+
+     dnf repolist --all
+
+#. Install ZFS packages::
+
+    dnf config-manager --disable zfs
+    dnf config-manager --enable zfs-kmod
+    dnf install -y zfs
+    #  if gpg import fails, add --nogpgcheck
+
+#. Load kernel modules::
+
+    modprobe zfs
+
+#. Install partition tool::
+
+    dnf install -y gdisk dosfstools