doc: Convert docs to Markdown language

Converts the majority of docs over to use the Markdown language for
pretty printing on GitHub. It's a rough first convertion without
exploiting the full potential of Markdown at this point. Section
titles and indentation are fixed as needed. Minimal docs interlinking
is added.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>