[Flang][Docs] Add a GettingStarted.md for build instructions

This patch fix first point of https://github.com/llvm/llvm-project/issues/60730.
https://flang.llvm.org/docs/ has no build instructions. This doc page is just
a copy of README page so that it can be accessible from website.

Differential Revision: https://reviews.llvm.org/D148070