venv changes require update to installation guide

[Nils Bruin]

I just noticed that 9.5beta9 (perhaps earlier already?) requires changes to the installation guide:

In https://doc.sagemath.org/html/en/installation/launching.html#setting-up-sagemath-as-a-jupyter-kernel-in-an-existing-jupyter-notebook-or-jupyterlab-installation

some explicit paths are mentioned. These are now

local/var/lib/sage/venv-python3.9.9/share/jupyter/kernels/sagemath/

rather than (what they were before): local/share/jupyter/kernels/sagemath

I suspect that with the venv-python3.9.9 labelling, this path will continue to need updating, so perhaps an alternative way of figuring out the appropriate path is preferred.

[Matthias Koeppe]

I have opened https://trac.sagemath.org/ticket/33111 for this