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>

doc/guides/conf.py

@@ -35,8 +35,12 @@
 from sphinx.highlighting import PygmentsBridge
 from pygments.formatters.latex import LatexFormatter
 
-project = 'DPDK'
+project = 'Data Plane Development Kit'
 
+if LooseVersion(sphinx_version) >= LooseVersion('1.3.1'):
+    html_theme = "sphinx_rtd_theme"
+html_logo = '../logo/DPDK_logo_vertical_rev_small.png'
+latex_logo = '../logo/DPDK_logo_horizontal_tag.png'
 html_add_permalinks = ""
 html_show_copyright = False
 highlight_language = 'none'