Add support for Alpine Linux

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

docs/Getting Started/Alpine Linux/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 latest extended variant of `Alpine Linux live image
+   <https://dl-cdn.alpinelinux.org/alpine/latest-stable/releases/x86_64/>`__ and boot from it.
+#. Login as root user.  There is no password.
+#. Configure Internet::
+
+     setup-interfaces -r
+     # example:
+     network interface: wlan0
+     WiFi name:         <ssid>
+     ip address:        dhcp
+     <enter done to finish network config>
+     manual netconfig:  n
+
+#. Configure SSH server::
+
+     setup-sshd
+     # example:
+     ssh server:        openssh
+     allow root:        "prohibit-password" or "yes"
+     ssh key:           "none" or "<public key>"
+
+   Configurations set here will be copied verbatim to the installed system.
+
+#. Set root password or ``/root/.ssh/authorized_keys``.
+
+#. Connect from another computer::
+
+    ssh root@192.168.1.19
+
+#. Throughout this guide, we use predictable disk names generated by udev::
+
+     apk add eudev
+     setup-udev
+
+   It can be removed after reboot with ``setup-devd mdev && apk del eudev``.
+
+#. 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'
+
+#. Configure NTP client for time synchronization::
+
+     setup-ntp chrony
+
+#. Pick a mirror from `Alpine Mirrors <https://mirrors.alpinelinux.org/>`__
+   and add it to package manager configuration::
+
+     tee -a /etc/apk/repositories <<EOF
+     https://dl-5.alpinelinux.org/alpine/latest-stable/community/
+     https://dl-5.alpinelinux.org/alpine/latest-stable/main/
+     EOF
+
+#. Install ZFS support and partition tool::
+
+    apk update
+    apk add zfs zfs-lts sgdisk e2fsprogs
+    modprobe zfs
+
+   Many errors about firmware will appear.  They are safe to ignore.
+
+#. Install bootloader for both legacy boot and UEFI::
+
+     apk add grub-bios grub-efi