remove Markdown dep (#189) #635

Job	Run time
Documentation	1m 21s
Julia 1.6 - ubuntu-latest - x64 - push	2m 2s
Julia 1.6 - windows-latest - x64 - push	12m 17s
Julia lts - ubuntu-latest - x64 - push	1m 52s
Julia lts - windows-latest - x64 - push	2m 38s
Julia 1 - ubuntu-latest - x64 - push	2m 58s
Julia 1 - windows-latest - x64 - push	3m 29s
Julia pre - ubuntu-latest - x64 - push	2m 55s
Julia pre - windows-latest - x64 - push	3m 54s
Julia nightly - ubuntu-latest - x64 - push	4m 34s
Julia nightly - windows-latest - x64 - push	5m 11s
	43m 11s

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