(https://github.com/ruby/rdoc/pull/1137)

This is an attempt to utilize RuboCop further.
RuboCop was added in https://github.com/ruby/rdoc/commit/9262fdd43a3a
but only a few rules have been enabled.
I believe we can utilize RuboCop more for better code quality,
especially with Lint cops.
This is the first step to enable other Lint cops.
This commit also exclude some auto generated files.

https://github.com/ruby/rdoc/commit/a53287fce0