Standardize Rule markup

[vinistock]

Continuing the work from #1389. This PR standardizes the Rule element to a consistent style and starts inheriting from Element.

[matzbot]

🚀 Preview deployment available at: https://970b30be.rdoc-6cd.pages.dev (commit: 34051e6)

[tompng]

Closing this because changing superclass of RDoc::Markup::Rule from Struct to another one might cause the same problem as #1535 (fixed by partially reverting superclass change in #1549)

class RDoc::Markup::Rule < Struct.new :weight

I think other standardize pull requests are safe