mkfs.bcachefs: add page (#13802)

pages/linux/mkfs.bcachefs.md

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+# mkfs.bcachefs
+
+> Create a `bcachefs` filesystem inside a partition.
+> More information: <https://bcachefs-docs.readthedocs.io/en/latest/mgmt-formatting.html>.
+
+- Create a `bcachefs` filesystem inside partition 1 on a device (`X`):
+
+`sudo mkfs.bcachefs {{/dev/sdX1}}`
+
+- Create a `bcachefs` filesystem with a volume label:
+
+`sudo mkfs.bcachefs {{-L|--fs_label}} {{volume_label}} {{/dev/sdX1}}`