Necessary GHOST api's for Demos Plugin

Whole idea of Demos plugin, could be, that users could have polls. But it would be possible to make a poll, that’ll share contributors among more phorums than one.

Where in docummentation I can find necessary API’s?

I am not entirely sure what you’re looking for specifically, but you can find the API documentation here:

Content API: Ghost Content API Documentation
Admin API: Ghost Admin API Documentation