doc: add Technical intro doc

Add the "Introduction of Project Acorn" doc.

Also adds improvements to the doc generation processes, content styles,
removed doxygen-generated API material.

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

doc/static/acrn-custom.css

@@ -5,6 +5,7 @@
    max-width: none;
 }
 
+/* (temporarily) add an under development tagline to the bread crumb */
 .wy-breadcrumbs::after {
    content: " (Content under development)";
    background-color: #FFFACD;
@@ -12,6 +13,16 @@
    font-weight: bold;
 }
 
+/* Make the version number more visible */
+.wy-side-nav-search>div.version {
+    color: rgba(255,255,255,1);
+}
+
+p.caption  {
+#    border-top: 1px solid;
+    margin-top: 1em;
+}
+
 /*  make .. hlist:: tables fill the page */
 table.hlist {
     width: 95% !important;