[documentation] Add a page about custom workspaces #1213

Job	Run time
setup_matrix	1m 57s
break (PartiallySeparableNLPModels, latest)	3m 17s
break (PartiallySeparableNLPModels, stable)	7m 57s
break (Percival, latest)	7m 2s
break (Percival, stable)	6m 50s
break (CaNNOLeS, latest)	6m 49s
break (CaNNOLeS, stable)	6m 22s
break (PartitionedVectors, latest)	36s
break (PartitionedVectors, stable)	1m 9s
break (DCISolver, latest)	5m 41s
break (DCISolver, stable)	5m 9s
break (JSOSolvers, latest)	11m 25s
break (JSOSolvers, stable)	11m 15s
break (PartiallySeparableSolvers, latest)	3m 41s
break (PartiallySeparableSolvers, stable)	3m 47s
break (RipQP, latest)	14m 23s
break (RipQP, stable)	14m 14s
break (AdaptiveRegularization, latest)	13m 59s
break (AdaptiveRegularization, stable)	13m 46s
break (FletcherPenaltySolver, latest)	26m 19s
break (FletcherPenaltySolver, stable)	27m 12s
upload	11s
	3h 13m 1s

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