docs/general/grub-migration: add grub migration guide

based on https://github.com/zbm-dev/zfsbootmenu/discussions/600#discussioncomment-8671690

closes #604

docs/general/grub-migration.rst

@@ -0,0 +1,49 @@
+Migration from GRUB
+===================
+
+While GRUB has very different requirements for filesystem layout, it is possible
+to migrate from GRUB to ZFSBootMenu.
+
+First, create a :doc:`portable installation </general/portable>` that will let
+you boot from a USB drive. This should give you all the tools you need to migrate
+``/boot`` into the root filesystem in such a way that ZFSBootMenu will recognize
+your boot environment. Af first, ZFSBootMenu will probably fail to recognize any
+boot environments and try dropping you into a recovery shell. Assuming your root
+filesystem is ``rpool/ROOT/debian`` and your boot filesystem is ``bpool/BOOT/debian``,
+you should be able to:
+
+1. Confirm that ``bpool`` and ``rpool`` are both imported. If not, you can manually
+   import each pool, or you can try running ``/libexec/zfunc import_pool`` which will
+   try to import both.
+2. Make sure the pools are writable::
+
+    set_rw_pool rpool
+    set_rw_pool bpool
+
+4. Use the ``mount_zfs`` helper to mount both the boot and root filesystems::
+
+    mount_zfs bpool/BOOT/debian
+    allow_rw=yes mount_zfs rpool/ROOT/debian
+
+   each call will print the path where the filesystem is mounted, which (in this
+   case) will be, respectively::
+
+    /zfsbootmenu/environments/bpool/BOOT/debian/mnt
+    /zfsbootmenu/environments/rpool/ROOT/debian/mnt
+
+5. Copy the contents of the boot filesystem to the ``boot`` subdirectory of the root::
+
+    cd /zfsbootmenu/environments/bpool/BOOT/debian/mnt
+    mkdir -p /zfsbootmenu/environments/rpool/ROOT/debian/mnt/boot
+    cp -a . /zfsbootmenu/environments/rpool/ROOT/debian/mnt/boot
+
+6. Make sure the boot pool won't be mounted at next boot::
+
+    zfs set canmount=noauto bpool/BOOT/debian
+
+7. Exit the recovery shell. You should now see your environment in the menu.
+
+After confirming this works, you can now install ZFSBootMenu in your EFI System Partition
+and add it to the boot order with ``efibootmgr`` (or use a bootloader like ``rEFInd`` to load it).
+
+Once everything works, you can destroy your ``bpool`` and remove GRUB from the ESP if you so choose.

docs/index.rst

@@ -56,6 +56,7 @@
   general/remote-access
   general/tailscale
   general/uefi-booting
+  general/grub-migration
 
 .. toctree::
   :caption: Installation Guides