doc: update install instructions for recent Ubuntu/Debian

[decalage2]

Several install instructions do not work as-is on Ubuntu/Debian (and possibly other Linux distros):

• to use olebrowse, the Tkinter package is required: sudo apt install python3-tk. This issue should also be detected by olebrowse to provide guidance instead of a cryptic exception.
• it looks like oletools cannot be installed system-wide by default, even in the user home. It needs to be installed in a virtual env (venv) and used from there. The user may create scripts to have tools available from any directory, but it needs to be done manually. :-/