Add distribution plots to `rand_distr` documentation #773

Job	Run time
Check doc	1m 9s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 42s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	1m 1s
test-ios	31s
test (macos-latest, x86_64-apple-darwin, stable)	1m 28s
test-miri	3m 50s
test (windows-latest, x86_64-pc-windows-gnu, stable)	2m 23s
test-no-std	16s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 54s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	2m 22s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 43s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 28s
	19m 47s

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