doc: add support for publishing versioned docs

add navigation to (manually) maintained list (in conf.py) of versioned
docs, and update generating and publishing processes to be
version-aware.

Adds a file to redirect root references to /latest folder now (since we
can't update the server redirects).  Might break some links to pages
within the site from external sites.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/_templates/breadcrumbs.html

@@ -0,0 +1,22 @@
+{% extends "!breadcrumbs.html" %}
+{% block breadcrumbs_aside %}
+
+<li class="wy-breadcrumbs-aside">
+   <div class="rst-versions" data-toggle="rst-versions" role="note" aria-label="versions">
+      <span class="rst-current-version" data-toggle="rst-current-version">
+         <span class="fa fa-book"> Project ACRN</span>
+         version: {{ current_version }}
+         <span class="fa fa-caret-down"></span>
+      </span>
+      <div class="rst-other-versions">
+         <dl>
+            <dt>Versions</dt>
+               {% for slug, url in versions %}
+               <dd><a href="{{ url }}">{{ slug }}</a></dd>
+              {% endfor %}
+         </dl>
+      </div>
+   </div>
+</li>
+{# {{ super() }} #}
+{% endblock %}