Clarify where repository rules should be defined

### Description of the feature request:

Add wording in the [Bazel rule deployment guidelines](https://docs.bazel.build/versions/master/skylark/deploying.html) clarifying where repository rules should be defined: `defs.bzl`, `repositories.bzl` or elsewhere?

### Feature requests: what underlying problem are you trying to solve with this feature?

Currently, the documentation says "rules" should be exposed in `defs.bzl`. There is a section titled "Repository rules", which one would expect says what to do about repository rules, but it only discusses introducing a single macro that defines all necessary dependencies. If I have a set of rules called `rules_foo`, which perhaps has a few build rules and several repository rules, it's unclear where the user should expect to find all the repository rules.

Start with the linked Bazel rule deployment guidelines, especially the existing rules and repository rules sections. Clarify where repository rules should be defined for a rules_foo set, including whether that is defs.bzl, repositories.bzl, or another location, and ensure the guidance is unambiguous.

Written by the indexing model from the issue text.