add NESTML 8 citation to documentation (#1154) #2395

Job	Run time
static_checks	39s
link_checker	30s
build_and_test_nest_desktop (master)	11m 29s
build_and_test_py_standalone	3m 3s
build_and_test_nest (v2.20.2)	27m 58s
build_and_test_nest_compartmental (master)	18m 20s
build_and_test_nest (v3.0)	21m 25s
build_and_test_nest (v3.8)	23m 14s
build_and_test_nest (master)	1h 0m 41s
	2h 47m 19s

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