mkosi: add page (#10721)

Co-authored-by: Jack Lin <blueskyson1401@gmail.com>
Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

pages/linux/mkosi.md

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+# mkosi
+
+> Tool for building modern, legacy-free Linux images.
+> Part of `systemd`.
+> More information: <https://manned.org/mkosi>.
+
+- Show current build configuration to verify what would be built:
+
+`mkosi summary`
+
+- Build an image with default settings (if no distribution is selected, the distribution of the host system is used):
+
+`mkosi build --distribution {{fedora|debian|ubuntu|arch|opensuse|...}}`
+
+- Build an image and run an interactive shell in a systemd-nspawn container of the image:
+
+`mkosi shell`
+
+- Boot an image in a virtual machine using QEMU (only supported for disk images or CPIO images when a kernel is provided):
+
+`mkosi qemu`
+
+- Display help:
+
+`mkosi help`