Backgrounder on plugin API

I did a little write-up on how plugin API works: https://github.com/eclipse/che/wiki/Writing-Theia-plugin-API. It might be helpful in theia documentation, but I’m not sure where.

[original thread by Thomas Mäder]

We have the arch section in docs: https://www.theia-ide.org/doc/

Maybe it would be a good place @svenefftinge ?

Yes, putting it into the documentation seems appropriate.