doc: Reorganize documentation site content

Take the existing ACRN technical documentation and reorganize its
presentation to be persona and use-case based, in preparation for adding
new scenario/use-case based architecture introduction and getting
started documents.

Introduce a more graphical home page and theme color tweaks.

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

doc/introduction/images/boot-flow.dot

@@ -1,6 +0,0 @@
-digraph G {
-   rankdir=LR;
-   bgcolor="transparent";
-   UEFI -> "acrn.efi" -> "OS\nBootloader" ->
-     "SOS\nKernel" -> "ACRN\nDevice Model" -> "Virtual\nBootloader";
-}