doc: Adds build-doc guidelines for Fedora and CentOS/RHEL.

Signed-off-by: Nilamdyuti Goswami <ngoswami@redhat.com>
Merge pull request #2932 from nilamdyuti/wip-doc-documenting-ceph

doc: Adds build-doc guidelines for Fedora and CentOS/RHEL.

Reviewed-by: Loic Dachary <ldachary@redhat.com>