RDoc 8.0.0 gem omits `doc/rdoc/example.rb`, which is referenced by the packaged markup documentation

Start with the gem packaging configuration and compare it with doc/rdoc/example.rb and doc/markup_reference/rdoc.rdoc. Reproduce the missing-file check against the built RDoc 8.0.0 gem, then verify that the packaged file is available to documentation generation and the referenced RDoc links resolve without warnings.

Description

The RDoc repository contains doc/rdoc/example.rb, and the markup reference
relies on the code objects defined by that file.

However, doc/rdoc/example.rb is absent from the released RDoc 8.0.0 gem.

The gem does contain:

doc/markup_reference/rdoc.rdoc

That document contains references including:

rdoc-ref:RDoc::Example#instance_method_example
rdoc-ref:RDoc::Example::EXAMPLE_CONSTANT

When documentation is generated from the installed gem and the markup
reference is included, these links cannot be resolved.

The upstream source file is:

https://github.com/ruby/rdoc/blob/v8.0.0/doc/rdoc/example.rb

The references are in:

https://github.com/ruby/rdoc/blob/v8.0.0/doc/markup_reference/rdoc.rdoc

Reproduction

require "rubygems"

spec = Gem::Specification.find_by_name("rdoc", "8.0.0")

example = File.join(spec.full_gem_path, "doc/rdoc/example.rb")
reference = File.join(
  spec.full_gem_path,
  "doc/markup_reference/rdoc.rdoc"
)

p File.file?(example)   # => false
p File.file?(reference) # => true

Generate documentation from the installed gem while including
doc/markup_reference/rdoc.rdoc.

The resulting warnings include:

`rdoc-ref:RDoc::Example#instance_method_example` can't be resolved
`rdoc-ref:RDoc::Example::EXAMPLE_CONSTANT` can't be resolved

Expected behavior

All source files required by the packaged documentation should also be
included in the gem.

Suggested fix

Add doc/rdoc/example.rb to the gem package manifest:

non_lib_files = [
  # ...
  "doc/rdoc/example.rb",
]

It should probably also be included explicitly in extra_rdoc_files so
RubyGems documentation generation parses it:

s.extra_rdoc_files << "doc/rdoc/example.rb"

A packaging test could assert that every source file required by the markup
reference is present in the built gem.