Expand plugins guide

[FichteFoll]

The current Plugins guide page has not been revised in 10 years or so. It does not mention anything that has been added since, i.e. async event handlers, ViewEventListener, Python 3.8 and the .python-version file, that the API is now available at module load time (since build xxxx), etc…

It needs a general revisit to update outdated information and should at least mention some of the new things, ideally in some more detail. I'm also thinking specifically about mentioning everything related for user interaction, i.e. add_regions, show_popup, rich completion items, status message, …