grub-reboot: add page (#9914)

* grub-reboot: add page

* Apply suggestions from code review

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: Jack Lin <blueskyson1401@gmail.com>

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: Jack Lin <blueskyson1401@gmail.com>

pages/linux/grub-reboot.md

@@ -0,0 +1,12 @@
+# grub-reboot
+
+> Set the default boot entry for GRUB, for the next boot only.
+> More information: <https://manned.org/grub-reboot>.
+
+- Set the default boot entry to an entry number, name or identifier for the next boot:
+
+`sudo grub-reboot {{entry_number}}`
+
+- Set the default boot entry to an entry number, name or identifier for an alternative boot directory for the next boot:
+
+`sudo grub-reboot --boot-directory {{/path/to/boot_directory}} {{entry_number}}`