[docs] Move code contribution from GettingStarted.rst to Contributing.rst

For code contribution, GettingStarted.rst duplicates information in Contributing.rst.
The dedicated Contributing.rst is a better place for code contribution, so move
the content there.

Notes:

* D41665 added `Contributing.rst`
* D110976 mentioned `git cherry-pick e3659d43d8911e91739f3b0c5935598bceb859aa` workaround

Reviewed By: cjdb, fhahn, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D129255