Usage Example for ThresholdMD re #9726

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Add usage example to MDHistoToWorkspace2D-v1.rst re #9726

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Merge branch 'feature/9726_algorithm_docs' into develop