Move mirror documentation from devel to sysadm

docs/architecture/index.rst

docs/architecture/mirror.rst

docs/index.rst

sysadm/images/mirror-architecture.svg

sysadm/mirror-operations/content-replayer.rst

sysadm/mirror-operations/deploy.rst

sysadm/mirror-operations/frontend.rst

sysadm/mirror-operations/graph-replayer.rst

sysadm/mirror-operations/index.rst

sysadm/mirror-operations/objstorage.rst

sysadm/mirror-operations/search.rst

sysadm/mirror-operations/storage.rst

sysadm/mirror-operations/vault.rst