[docs] Add a performance doc

[ttheodor]

Summary

Add a document which describes:

1. GEMM performance comparison
2. an experiment that measures the compile time impact of enabling Polly when compiling LLVM+Clang+Polly.

[grosser]

LGTM. However, can you make sure this patch is referenced from the Polly documentation index.

[ttheodor]

Switch to a general "Performance" document that also includes a GEMM example.

docs/CompileTimeImpactOfPolly.rst

+==================================================
+Compile Time Impact of Polly
+==================================================
+Experimental Setup
+------------------------
+Clang+LLVM+Polly are compiled using Clang on a Intel(R) Core(TM) i7-7700 based system. The experiment is repeated twice: with and without Polly enabled in order to measure its compile time impact.
+
+The following versions are used:
+
+
+- Polly (git hash 0db98a4837b6f233063307bb9184374175401922)
+- Clang (git hash 3e1d04a92b51ed36163995c96c31a0e4bbb1561d)
+- LLVM  git hash 0265ec7ebad69a47f5c899d95295b5eb41aba68e)
+
+`ninja <https://ninja-build.org/>`_ is used as the build system.
+
+For both cases the whole compilation was performed five times and the median measurements are reported.
+
+Results
+--------
+
+The median compile time without Polly enabled is 967 seconds and with Polly enabled it is 981 seconds. The overhead is 1.4%.