Resolve "Quick clarifications for current plugin docs"

Closes #1990

Some clarifications for plugin setup (especially for parsers) and starting to move towards an actual How-to style. Need to harmonize with #1989 (closed)

@himanel1 you have the priority here, my understanding from Markus is that you are well underway with your docs on the new plugin mechanism. In that case, I just wait for you to finish and then re-integrate any relevant changes after. Please let me know your timeline if possible (I am away from May 1-13 in any case, so probably it waits until after). Feel free to take a look if it might be useful, or just ignore.