Update harmonised variables guidance on analysts' guide

**Is your feature request related to a problem? Please describe.**

The guidance on harmonised variables needs to be updated. This is because some categories are already missing from establishment variables from the guidance. Also, I am working on adding categories in the screener for establishment type group, SEN provision and ethnicity so this issue will be ongoing until that project ends.

**Describe the solution you'd like**

Update the guidance to include the missing categories and also update it to include the categories I'm adding in.

**Describe alternatives you've considered**
Sharing the new updates to each team separately instead of centrally. Not efficient.

**Additional context**

This will be an ongoing issue until the KS2 API project ends but I'm using this ticket to keep track of the changed variables.