docs: Mount MicroCeph backed Block Devices

Hi,

While following this doc, after installation of ceph-common packages, "ceph" command is not working as expected anymore. It is looking under /etc/ceph/ directory for conf and keyring files. So to use it as expected, either "microceph.ceph" command needs to be used or ceph.conf and keyring files should be copied as:

ubuntu@mc3:~$ sudo cp /var/snap/microceph/current/conf/ceph.conf /etc/ceph/
ubuntu@mc3:~$ sudo cp /var/snap/microceph/current/conf/ceph.keyring /etc/ceph/

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*Reported from: https://canonical-microcloud.readthedocs-hosted.com/en/latest/microceph/tutorial/mount-block-device/*