doc: add a custom 404 page

Add a site-wide custom error page for broken references within the site.
Besides giving a better user experience, it's also easier to track
broken links to the site using google analytics (looking for hits to the
404.html page and noticing the referrer.

Note we only "publish" the custom 404.html when publishing the latest
documents (not for released versions) so we always get the latest
left-navigation menu when a 404 error occurs.

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

doc/404.rst

@@ -0,0 +1,24 @@
+:orphan:
+
+.. _page-not-found:
+
+Sorry, Page Not Found
+#####################
+
+.. image:: images/ACRN-fall-from-tree-small.png
+   :align: left
+   :scale: 50%
+
+|
+| Sorry, the page you requested was not found on this site.
+|
+|
+| Please check the address for misspellings.
+| It's also possible we've removed or renamed the page you're looking for.
+|
+| Please try using the navigation links on the left of this page to navigate
+| the major sections of our site, or use the document search box.
+
+.. raw:: html
+
+   <div style='clear:both'></div>

doc/Makefile

@@ -85,6 +85,9 @@ publish:
 	cp scripts/publish-README.md $(PUBLISHDIR)/../README.md
 	cp scripts/publish-index.html $(PUBLISHDIR)/../index.html
 	cp scripts/publish-robots.txt $(PUBLISHDIR)/../robots.txt
+	ifeq ($(RELEASE),latest)
+	  cp _build/html/404.html $(PUBLISHDIR)/../404.html
+	endif
 	cd $(PUBLISHDIR)/..; git add -A; git commit -s -m "publish $(RELEASE)"; git push origin master;