Added establishment types reference data

[rmbielby]

Overview of changes

Similar to the SEN data guidance update, this presents the first iteration of standardisation of the school type fields as reviewed by the data harmonisation champions.

Why are these changes being made?

These are part of the ongoing effort to establish standardised data guidance for publication teams to follow.

Detailed description of changes

Standard field names and items are outlined for:

Establishment type group
Establishment governance
Education phase
Establishment type
Provider type

Checklist before requesting a review

• I have checked the contributing guidelines
• I have checked for and linked any relevant issues that this may resolve
• I have checked that these changes build locally
• I understand that if merged into main, these changes will be publicly available

[cjrace]

Like this now I think that's much cleaner to understand at a glance, one minor point about in page links then will be happy to approve