Abseil clang-tidy Markdown port

Check name	Prev	Now
abseil-duration-comparison	LLVM documentation	markdown version
abseil-duration-conversion-cast	LLVM documentation	markdown version
abseil-duration-division	LLVM documentation	markdown version
abseil-duration-factory-scale	LLVM documentation	markdown version
abseil-duration-subtraction	LLVM documentation	markdown version
abseil-duration-unnecessary-conversion	LLVM documentation	markdown version
abseil-faster-strsplit-delimiter	LLVM documentation	markdown version
abseil-string-find-startswith	LLVM documentation	markdown version
abseil-string-find-str-contains	LLVM documentation	markdown version
abseil-time-subtraction	LLVM documentation	markdown version
abseil-unchecked-statusor-access	LLVM documentation	markdown version
abseil-upgrade-duration-conversions	LLVM documentation	markdown version