grub-editenv: add page (#11218)

* grub-editenv: add page

---------

Co-authored-by: Magrid <magrid0@proton.me>
Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/linux/grub-editenv.md

@@ -0,0 +1,20 @@
+# grub-editenv
+
+> Edit GRUB environment variables.
+> More information: <https://www.gnu.org/software/grub/manual/grub/grub.html>.
+
+- Set a default boot entry (Assuming the boot entry already exists):
+
+`grub-editenv /boot/grub/grubenv set default={{Ubuntu}}`
+
+- Display the current value of the `timeout` variable:
+
+`grub-editenv /boot/grub/grubenv list timeout`
+
+- Reset the `saved_entry` variable to the default:
+
+`grub-editenv /boot/grub/grubenv unset saved_entry`
+
+- Append "quiet splash" to the kernel command line:
+
+`grub-editenv /boot/grub/grubenv list kernel_cmdline`