Fix a number of things with the generated docs, including translating triple-backtick code blocks into a form doxygen understands.

doc/fixup_markdown.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+#
+# Doxygen markdown doesn't support triple-backticks like github does.
+# Convert all of those to space-prefixed blocks instead.
+#
+awk '/```/ { prefix=!prefix; print ""; next; } { if (prefix) { printf "    "; } print $0; } ' "$@"