Improve plugins documentation and create more examples

This is an Nlnet milestone. “Improve plugins documentation and create more examples”

That will go on the docs: Plugins | Karrot Docs

Background:

Enhance self-hosting of instances

Building upon our previous work that added the possibility to develop plugins, which is of particular interest to self-hosted instances, we want to continue work on that to improve the experience, now we have some groups writing their own plugins. Based on user feedback we understand the need for more documentation and examples, an easier way to package and install plugins, and ability to publish/install plugins in a way that updates can be handled easily.