[Polly] Clean up Polly's getting started docs.

This patch removes the broken bash scipt (polly.sh) and fixes the broken setup
instructions in get_started.html. It also adds instructions for using Ninja and
links to the LLVM getting started page.

Reviewed By: Meinersbur, InnovativeInventor

Differential Revision: https://reviews.llvm.org/D111685

GitOrigin-RevId: 5f668bba55742c081947011e37d5a7386ca58d01
2 files changed