[docs] Add a new tutorial that talk about how to make a change to llvm

[docs] Add a new tutorial that talk about how to make a change to llvm

This tutorial will guide you through the process of making a change to LLVM, and contributing it back to the LLVM project.
We'll be making a change to Clang, but the steps for other parts of LLVM are the same. Even though the change we'll be making is simple,
we're going to cover steps like building LLVM, running the tests, and code review. This is good practice, and you'll be prepared for making larger changes.

Authors: @meikeb , @gribozavr
Commit: Zhiqian Xia

PS - This is a duplicate revision of https://reviews.llvm.org/D100714 which was actually used for patch review.

Reviewed By: kuhnel

Differential Revision: https://reviews.llvm.org/D108267