The content from architecture.rst was moved to install/get_started.rst
and merged under a new heading "Overview of Ironic".
To avoid breaking links and search engine results pointing to the old location,
this patch restores architecture.rst with a redirect notice to
the new location.
Partial-Bug: #2072353
Change-Id: I950713434132acc5b225a39c0e4cdd724691f9e6