chore: add markdownlint config #53

Job	Run time
ci / Change Detection	7s
ci / Markdownlint	14s
ci / StyLua	-1s
ci / Docs	-1s
ci / Detect Metadata	-1s
ci / Setup Matrix	-1s
ci / Release Please	-1s
ci / Run Tests (${{ matrix.os }}, ${{ matrix.lua-version }})	-1s
ci / Publish to LuaRocks	-1s
ci / Luacheck	-1s
	13s

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