improve appearance of quotation

This patch improves the appearance of the quotation of a source code
and a console log for latex/pdf.

conf.py:
- added configuration for listings module to latex_elements['preamble'].

switching_hub.rst:
- replace a literal-block with a raw latex code to use the listings.
- put a quotation of a source code between "\begin{sourcecode}" and
  "\end{sourcecode}". put a console log between "\begin{console}"
  and "\end{console}".
- use "\lstinputlisting" of latex instead of "literalinclude" of
  sphinx to include an external source file.

index.rst:
- remove unnecessary description.

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>