Switch to official releases of katex and update doc for installing katex. (#27758)
Summary:
katex is a deprecated package in Ubuntu and has been removed in recent
releases of Debian. Use npm instead.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27758
Differential Revision: D17891039
Pulled By: ezyang
fbshipit-source-id: 53de6e14b2638298e5b61996dcd7ba8de02420a3
