Add basic docs, expand filldist/arraydist docs #213

Job	Run time
test (1.6, ubuntu-latest, x64, Others)	2m 24s
test (1.6, ubuntu-latest, x64, ForwardDiff)	35m 1s
test (1.6, ubuntu-latest, x64, Tracker)	17m 20s
test (1.6, ubuntu-latest, x64, ReverseDiff)	35m 2s
test (1.6, ubuntu-latest, x64, Zygote)	30m 51s
test (1, ubuntu-latest, x64, Others)	2m 43s
test (1, ubuntu-latest, x64, ForwardDiff)	35m 0s
test (1, ubuntu-latest, x64, Tracker)	12m 10s
test (1, ubuntu-latest, x64, ReverseDiff)	13m 9s
test (1, ubuntu-latest, x64, Zygote)	23m 47s
test (1, macOS-latest, x64, Others)	3m 32s
test (1, macOS-latest, x64, ForwardDiff)	35m 1s
test (1, macOS-latest, x64, Tracker)	13m 8s
test (1, macOS-latest, x64, ReverseDiff)	14m 28s
test (1, macOS-latest, x64, Zygote)	20m 27s
	4h 54m 3s

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