Add chapter to create own backend

[stevie-sy]

Some time ago I started to create an own backend (with tipps from @nilmerg some time ago 🚀 ). In the issue Icinga/icingaweb2#5265, where I dicsribed a bug, @nilmerg also wrote some ways to provide the backend in the own module.

So that this tipps and infos doesn't get lost, it would be great to have this infos in the training module as well - just to be documented somehwere. I know this is an advanced topic. I think it's enough to have this in an small chapter. Just if somebody else is interested into this topic.

[martialblog]

Hi, thanks for the hint. We might be able to include it here.

Not sure yet where to situate the content. But feel free to open a PR with a write-up.