[llvm][TableGen][Jupyter] Record current python when kernel is installed

Previously the kernel.json would always point to `python3` even if you
installed using a python from a virtualenv. This meant that tools like VSCode
would try to run the kernel against the system python and fail.

Added a note to the readme about it. I've removed the need to
add to PYTHONPTHON as well, turns out it wasn't needed.

This fixes an issue reported in https://discourse.llvm.org/t/tablegen-the-playground-ipynb-file-is-not-working-as-expected/71745.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D154351