Added note about katex install for building docs (#81550)
Currently, the version of katex that npm installs
and the version of nodejs that is installed by conda
from conda-forge are incompatible. This note describes
which version of katex to install to avoid this
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81550
Approved by: https://github.com/Lezcano, https://github.com/kit1980
