Add Code of Conduct page

[s-ben]

Port Code of Conduct (CoC) that we ratified in Town Hall into new page. Imagining it would go best under 'Join Our Community' top-level heading.

https://docs.google.com/document/d/1yu7objJo-oIkZSPLWCHvmV7-mZQOZ_BfK8JL4loq6NE/edit