Relax the "no religion or politics" rule

I was a bit surprised when I first read this rule in the handbook, and would like to politely challenge it :)

I assume this was added before we had a formal Code of Conduct, and think the CoC should cover any concerns we might have about this (which is also why I added a link under the "Diversity" heading).

The changed text is just a draft, any suggestions to improve it are very welcome! If it makes more sense we could also drop the whole paragraph, or move some of it into the CoC.