Oncosts: Document how on-cost calculations are made

We want to document and agree with HR how the on-cost calculation is completed. This is to ensure that we align the calculation made by the on-cost webapp with the calculation made by the HR reporting team which currently populates the on-cost data in the HR Drupal site.

Acceptance criteria

• We have reviewed and documented the on-cost calculation which is currently completed in the on-cost library - this could go into a page within the new webapp, as none of this information is sensitive, and it would allow users to understand how values are interpreted
• We have provided this documentation to the HR Reporting team for their review and comment on whether this matches the calculations they use to generate the on-cost values displayed on the HR Drupal site