Add metadata links to the gemspec

[tas50]

Summary

Adds a metadata block to chefspec.gemspec so rubygems.org/gems/chefspec renders the standard sidebar links (source, changelog, issues, docs) instead of just the homepage.

s.metadata = {
  "source_code_uri"   => "https://github.com/chef/chefspec",
  "changelog_uri"     => "https://github.com/chef/chefspec/blob/main/CHANGELOG.md",
  "bug_tracker_uri"   => "https://github.com/chef/chefspec/issues",
  "documentation_uri" => "https://github.com/chef/chefspec/blob/main/README.md",
}

Notes

• homepage_uri is intentionally omitted — it would duplicate the existing s.homepage and source_code_uri (RubyGems warns when two metadata keys share a URL).
• rubygems_mfa_required was deliberately left out: it requires an OTP on push and could break the automated Expeditor publish pipeline. That's a maintainer/release-policy decision better made separately.

Testing

• Gem::Specification.load("chefspec.gemspec").validate → valid, no warnings.
• cookstyle --chefstyle -c .rubocop.yml chefspec.gemspec → no offenses.