See discussion in #3494

It would be good to open the page in the default browser, when typing mkdocs serve (as happens when running the jupyter notebook command.

Various points

1. This should be done once, not every time a change is detected.
2. Make it work by command-line option and config file : opt-in (initially) and then opt-out (if successful).
3. I have experimented with code, and one of the problems I met was with async: the time it might take for the page to load on the browser, versus the time it takes for the server to start.