Is there a poly equivalent of something like the Magna Carta?

They mostly don't. Poly people think they do, but you see far, far more relationship volatility in polyerotic relationships than you do in monogamous.

Edit: I see that I'm getting downvoted by the people that are in non-monogamous relationships. Fact is that when you talk to sex-positive sex and relationship counselors, they will almost universally say that functional polyerotic relationships are the equivalent of post-doctoral work, while most people have relationship abilities equivalent to a barely-literate middle school level. It's not that multiamorous relationships are bad or wrong, or that the people that engage in them are wretched examples of humans (...although there are certainly more than a few of those) or anything like that, but to be functional that type of relationship requires a far greater level of self-awareness and honesty than most people are capable of. Hence the reason that they tend to be so volatile; more moving parts, more chances to fuck up.

In my personal experience I have found that most multiamorous relationships are more casual and less emotionally intimate (e.g., more shallow) than monogamous relationships. The people I have personally observed, including my own partners, have had less time to spend with any single person, and were more likely to jettison relationships rather than putting in the hard work to fix problems.