Adding YARD document support to RDoc

Background

I implemented a plugin system for RDoc in https://github.com/ruby/rdoc/pull/1321 and YARD parsing plugin is implemented.
In the discussion with @st0012, @kou and @vinistock we decided that we should implement YARD parsing feature as a standalone feature without plugin.
The note exists in https://github.com/ruby/rdoc/discussions/1257#discussioncomment-12882912

Steps

There are some steps to have effective YARD parsing feature in RDoc.

• Adding basic framework to support YARD style document (RDoc::Yard class or similar)
• Adding support of YARD tags that already works with current RDoc such as @yield and @private
  • @yield
  • @private
• Adding support of YARD tags that needs simple modification of RDoc such @deprecated
  • Adding :deprecated directive
  • Adding @deprecated
• Adding features such as type support to utilize information from YARD
  • @return
  • @param
  • others (will be edited later)
• Adding support of YARD directives and macros
  • @!attribute
  • others (will be edited later)

I will implement these step by step.

Note

The idea of plugin system is not completely abandoned but postponed.

Start by reviewing pull request #1321 and the linked discussion to understand why the plugin approach was postponed. Use the first unchecked roadmap item to define a focused contribution; completion should cover one agreed YARD capability, such as the basic framework or a listed tag, rather than the entire feature set.