docs: add complete build123d + visualization demo via ocp_vscode "standalone" mode

[jdegenstein]

Now that ocp_vscode supports use in a regular browser (and not strictly bound with VSCode), it is much easier to explain how one could simply:

1. pip install build123d ocp_vscode
2. execute python -m ocp_vscode
3. open http://127.0.0.1:3939/viewer in a suitable browser
4. execute a python file with necessary build123d and ocp_vscode imports and show statement

As a way to get users "up and running quickly" I propose something similar to the above is added to the build123d docs (with appropriate links/caveats back to the ocp_vscode repo/docs). Further I propose that a screenshot is added to show what this could look like (maybe with vim + chrome, or something similar).

[gumyr]

+1

[jdegenstein]

Screenshot for the docs [using Ubuntu 20.04, gedit, gnome-terminal, ipython and firefox]:

[gumyr]

Nice