Document disclosure control procedures

[ChrisBeeley]

There's a longer conversation about disclosure control going on but for now would be good to document the current practice and why we have adopted it.

There are many practices adopted even within the same organisations but the HES disclosure rules seem the most relevent https://digital.nhs.uk/data-and-information/data-tools-and-services/data-services/hospital-episode-statistics/disclosure-control-methodology-for-hospital-episode-statistics-and-emergency-care-data-set

[ChrisBeeley]

The analysis we carried out previously is described here https://github.com/The-Strategy-Unit/nhp_analysis/tree/main/2024-05-14_small-count-checks