Add basic docs, expand filldist/arraydist docs #212

Job	Run time
test (1.6, ubuntu-latest, x64, Others)	2m 21s
test (1.6, ubuntu-latest, x64, ForwardDiff)	1h 43m 11s
test (1.6, ubuntu-latest, x64, Tracker)	16m 40s
test (1.6, ubuntu-latest, x64, ReverseDiff)	54m 50s
test (1.6, ubuntu-latest, x64, Zygote)	31m 21s
test (1, ubuntu-latest, x64, Others)	5m 2s
test (1, ubuntu-latest, x64, ForwardDiff)	52m 15s
test (1, ubuntu-latest, x64, Tracker)	12m 7s
test (1, ubuntu-latest, x64, ReverseDiff)	13m 14s
test (1, ubuntu-latest, x64, Zygote)	23m 53s
test (1, macOS-latest, x64, Others)	5m 36s
test (1, macOS-latest, x64, ForwardDiff)	3h 7m 40s
test (1, macOS-latest, x64, Tracker)	10m 49s
test (1, macOS-latest, x64, ReverseDiff)	11m 46s
test (1, macOS-latest, x64, Zygote)	20m 9s
	9h 10m 54s

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