docs: vitepress conversion #846

Job	Run time
cs-stan / Coding Standard & Static Analysis	37s
testsuite / testsuite-linux (8.3, highest, false)	35s
testsuite / testsuite-linux (8.2, highest, false)	32s
testsuite / testsuite-linux (8.5, highest, false)	1m 15s
testsuite / testsuite-linux (8.2, lowest, false)	34s
testsuite / testsuite-linux (8.4, highest, false)	1m 16s
	4m 49s

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