As with the cells v2 docs before this, we have a number of architecture
focused documents in tree. The 'user/architecture' guide is relatively
up-to-date but is quite shallow, while the 'admin/arch' guide is
in-depth but almost a decade out-of-date, with references to things
like nova's in-built block storage service. Replace most of the latter
with more up-to-date information and the merge the former into it,
before renaming the file to 'admin/architecture'.
Change-Id: I518bb5d586b159b4796fb6139351ba423bc19639
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
ef4d759e87