Delete duplicated technical content from contribution_guide.rst (#18628)

Delete duplicated technical content from contribution_guide.rst (#18628)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18628
ghimport-source-id: d94b81a6f303883d97beaae25344fd591e13ce52

Stack from [ghstack](https://github.com/ezyang/ghstack):
* #18629 Provide flake8 install instructions.
* **#18628 Delete duplicated technical content from contribution_guide.rst**

There's useful guide in contributing_guide.rst, but the
technical bits were straight up copy-pasted from CONTRIBUTING.md,
and I don't think it makes sense to break the CONTRIBUTING.md
link.  Instead, I deleted the duplicate bits and added a cross
reference to the rst document.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Differential Revision: D14701003

fbshipit-source-id: 3bbb102fae225cbda27628a59138bba769bfa288