Expand documentation about visibility #8982

At the moment, we have this documentation: https://docs.bazel.build/versions/0.28.0/be/common-definitions.html#common-attributes

I think it's an important topic, it should be explained in a separate section (not in the attribute documentation).

Things to mention:

• Why visibility is important (especially as a code base grows)
• Discourage public visibility
• Visibility of rule dependencies (Add support for rule visibility for private tools/dependencies. #7377)
• Visibility of files, with and without exports_files (Data dependencies implicitly export files #2835)
• Visibility of bzl files