[docs] Fix broken links in `contribution_guide.rst` and `governance.rst` (#37820)
Summary:
Fix https://github.com/pytorch/pytorch/issues/37716
Fix three broken links in the documentation:
- [PyTorch Governance](https://pytorch.org/docs/source/community/governance.rst) in the [Contribution Guide page](https://pytorch.org/docs/master/community/contribution_guide.html#the-pytorch-contribution-process)
- [PyTorch Governance | Persons of Interest](https://pytorch.org/docs/source/community/persons_of_interest.rst) under the [Core Developer section](https://pytorch.org/docs/master/community/governance.html#core-developers)
- [PyTorch Contributor Guide](https://pytorch.org/docs/source/community/contribution_guide.rst) under the [FAQ session of the Governance Page](https://pytorch.org/docs/master/community/governance.html#faq)
The old link leads to the `.rst` source file, which does not exist on the server.
It's now fixed using the [document cross-referencing syntax](https://www.sphinx-doc.org/en/1.8/usage/restructuredtext/roles.html#cross-referencing-documents)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37820
Differential Revision: D21414579
Pulled By: mruberry
fbshipit-source-id: ecf6de9317ce93f70205cbfe97a3bdd54e635fe5
