Pdct 1095 move document type to taxonomy

[katybaulch]

What's changed

• Moved the document type into _document sub taxonomy
• Updated tests

Proposed version

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[linear]

PDCT-1095 Move document-type to taxonomy

[diversemix]

Fabulous - one really petty point... the PR title should probably be "add" (although the end result of the ticket is to move) 😁