config: Use golden for TestRuntimeConfig_Sanitize
To reference decode.HookWeakDecodeFromSlice instead. Also removes a step from the adding config fields checklist which is no longer necessary.
* Add contributing dir with Config file checklist and modify contribution guides * Apply suggestions from code review Co-Authored-By: Chris Piraino <cpiraino@hashicorp.com> Co-authored-by: Chris Piraino <piraino.chris@gmail.com>