Yes; Epic participates in the CareEverywhere network which has a gateway to the CareQuality network, thus supporting decent interoperability with other hospitals and clinics using a variety of EHRs.

Now, how much they share is potentially open to debate.

Is there any public documentation on Epic and how one can interact with it?

Yes, it works in practice. The trouble is more with the customers than the vendor. Some customers are still using obsolete versions that are missing the latest standards-based open interoperability features. Or they just haven't enabled those features.