Document `iris.site_configuration`?

[trexfeathers]

📚 Documentation

This is a pretty valuable feature. I have heard suggestions that it might be undocumented because it's a backdoor that we don't want to advertise, but I personally think it's a good thing - either document or remove IMO.

See here for some details of what it does: https://github.com/SciTools/iris/issues/5255#issuecomment-2125300801