• Update sphinx doc year to 2023
• Fix some typos in sphinx doc
• Add symbolic links to two jupyter-notebooks examples

Remark:
make html was not completely successful.
I got an error related to use_numpy_matrix.

"WARNING: autodoc: failed to import function 'use_numpy_matrix' from module 'control'; the following exception was raised:"

I guess sphinx uses an old version of control for autodoc? ==> doc/conf.py ==> git describe?
However, I was not able to find the reason.