Remove C++ docs contributing page (#31908)
Summary:
Stacked PRs
* **#31908 - Remove C++ docs contributing page**
* #31905 - Add doc previewing instructions
We should have 1 source of truth for contribution instructions (CONTRIBUTING.md).
This PR moves the instructions from the C++ doc pages there instead of having its
own separate page.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31908
Pulled By: driazati
Differential Revision: D19296366
fbshipit-source-id: c1daf004259342bd09e09dea3b80e34db47066ec
