Improve Documentation for Building Documenation

[jzf2101]

Very meta. See #1749

Some relevant info:
docs can only be built w python 3
page in read the docs on building docs is blank-
info should go on this page https://jupyter.readthedocs.io/en/latest/contrib_docs/doc-build.html
see also as reference https://readthedocs.org/projects/jupyter-notebook/builds/5880632/
also note installing pandoc w brew

[clesleycode]

I'll take care of this!

[jtpio]

Closing the documentation for building the docs has been revamped on the latest main: https://jupyter-notebook.readthedocs.io/en/latest/contributing.html#documentation