Move yard opts file into root folder

Although there is the `--yardopts` file option and we only build docs
for the `/core` (yet), I think it is best to share the options for all
subfolders.

Also does not require the gem on boot as we only run yard via command
line anyway.

Gemfile

@@ -59,7 +59,7 @@ end
 gem 'rspec_junit_formatter', require: false, group: :ci
 
 # Documentation
-gem 'yard'
+gem 'yard', require: false
 
 custom_gemfile = File.expand_path('Gemfile-custom', __dir__)
 eval File.read(custom_gemfile) if File.exist?(custom_gemfile)