Add basic docs, expand filldist/arraydist docs (#268) #216

Job	Run time
test (1.6, ubuntu-latest, x64, Others)	4m 30s
test (1.6, ubuntu-latest, x64, ForwardDiff)	2h 1m 24s
test (1.6, ubuntu-latest, x64, Tracker)	16m 0s
test (1.6, ubuntu-latest, x64, ReverseDiff)	55m 58s
test (1.6, ubuntu-latest, x64, Zygote)	34m 27s
test (1, ubuntu-latest, x64, Others)	3m 45s
test (1, ubuntu-latest, x64, ForwardDiff)	1h 3m 57s
test (1, ubuntu-latest, x64, Tracker)	16m 8s
test (1, ubuntu-latest, x64, ReverseDiff)	17m 8s
test (1, ubuntu-latest, x64, Zygote)	28m 10s
test (1, macOS-latest, x64, Others)	6m 56s
test (1, macOS-latest, x64, ForwardDiff)	3h 30m 8s
test (1, macOS-latest, x64, Tracker)	16m 47s
test (1, macOS-latest, x64, ReverseDiff)	16m 42s
test (1, macOS-latest, x64, Zygote)	34m 35s
	10h 46m 35s

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