Update `coder/coder` so that any references to the `coder/modules` repo use new Hub repo instead

This is technically spike work, but hopefully (1) it'll be a tiny spike, and (2) there actually won't be much work to do at all.

Once the Hub repo has been created, we'll want to update `coder/coder` to reference that (not just for the code, but for any README links as well). That might include removing the starter templates from `coder/coder` completely – need to decide how to handle that.