Link existing examples to Ohai docs

[vinyar]

Page: https://docs.chef.io/ohai_custom.html

Additional example suggestions for external links: Ohai example of executing arbitrary Powershell inside Ohai plugin: https://github.com/chef/ohai/blob/13485c17da688654890047d1952d9f71e735d162/lib/ohai/plugins/powershell.rb

Ohai example for collecting info on windows: https://github.com/chef-cookbooks/windows/blob/master/files/dism_features.rb

@smurawski for additional examples.