Porting the example illustrating Polly from HTML to reStructuredText

http://polly.llvm.org/example_manual_matmul.html

 which illustrates individual
passes of Polly, has been ported to reStructuredText and necessary changes have
been made to the configuration files used by SPHINX to include the new source as
a part of the documentation.

Contributed-by: Singapuram Sanjay Srivallabh <singapuram.sanjay@gmail.com>

Differential Revision: https://reviews.llvm.org/D25163

llvm-svn: 294735