Facility has to be set to 'ILL' in testsetup.

Re #20453

Added a real IN4 dataset to doc test data dirs, and updated the
DirectILL* algorithms' docs to use that instead of the 'fake' IN4
workspaces. Also, upgraded the DirectILL concepts page to use the real
data.

Re #20453

This reverts commit 67938cfc.
I realised that this would be a good exercise for the next person
to take over isis_powder, so have left the commit in for reference

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Re #21901

Re #21901

Re #21280

Refs #21378

Refs #21268

Refs #21294