improve validation and examples for code host site config

Created by: sqs

The github[].url field in site config is required. It was a reasonable assumption that it defaulted to https://github.com if unset (in fact, I assumed that, and it was true at some point). But it didn't, and that caused #522.

I considered fixing this by actually ensuring that the default https://github.com is used. I decided against that because we had that behavior before and it led to this bug, and being explicit and consistent with other code host config schemas is good.

This will help avoid the problem in https://github.com/sourcegraph/sourcegraph/issues/522#issuecomment-433249767 (which would have been MUCH easier to spot if the url field had been marked as required in the JSON Schema).