doc: change theme of guides

Change the Sphinx default theme from "alabaster" to the ReadTheDocs
theme. See for example:
http://dpdk.readthedocs.org/en/latest/

This looks better for technical do

doc: change theme of guides

Change the Sphinx default theme from "alabaster" to the ReadTheDocs
theme. See for example:
http://dpdk.readthedocs.org/en/latest/

This looks better for technical documentation and in particular
it has a 80 char wide verbatim block rendering.

Also add logos for the html and latex docs.

Signed-off-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>

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