Add some more Github standardization

[ecooper]

Problem

We're missing a few small bits on standard GitHub usage:

• CODEOWNERS
• CONTRIBUTING.md

And, while we're here, the Jira reference existed in the pr template.

Solution

• Add CODEOWNERS: Just default to @fauna/developer-experience for now
• Move DEV-README.md to CONTRIBUTING.md: This will eventually be the same thing. CONTRIBUTING.md is the stardard approach. We'll reconcile README.md when v3 is ready to be v4.
• Update PR template: Remove the jira ticket reference.