docs: a non-blocking error in YARD generation

[jaredcwhite]

From: https://gist.github.com/cmer/5e835483015f361aa1f0cf55df3f3ee9

[error]: Unhandled exception in YARD::Handlers::Ruby::ExtendHandler:
  in `lib/bridgetown-core.rb`:265:

	265: Psych.extend Bridgetown::CoreExt::Psych::SafeLoadFile unless Psych.respond_to?(:safe_load_file)

[error]: ProxyMethodError: Proxy cannot call method #mixins on object 'Psych'

            raise ProxyMethodError, "Proxy cannot call method ##{meth} on object '#{path}'"
            ^^^^^
[error]: Stack trace:
	/Users/carl/.asdf/installs/ruby/3.1.0/lib/ruby/gems/3.1.0/gems/yard-0.9.27/lib/yard/code_objects/proxy.rb:194:in `rescue in method_missing'
	/Users/carl/.asdf/installs/ruby/3.1.0/lib/ruby/gems/3.1.0/gems/yard-0.9.27/lib/yard/code_objects/proxy.rb:191:in `method_missing'
	/Users/carl/.asdf/installs/ruby/3.1.0/lib/ruby/gems/3.1.0/gems/yard-0.9.27/lib/yard/handlers/ruby/mixin_handler.rb:37:in `process_mixin'
	/Users/carl/.asdf/installs/ruby/3.1.0/lib/ruby/gems/3.1.0/gems/yard-0.9.27/lib/yard/handlers/ruby/extend_handler.rb:19:in `process_mixin'
	/Users/carl/.asdf/installs/ruby/3.1.0/lib/ruby/gems/3.1.0/gems/yard-0.9.27/lib/yard/handlers/ruby/mixin_handler.rb:12:in `block (2 levels) in <class:MixinHandler>'
	/Users/carl/.asdf/installs/ruby/3.1.0/lib/ruby/gems/3.1.0/gems/yard-0.9.27/lib/yard/handlers/ruby/mixin_handler.rb:10:in `each'