Catch broken markdown link formatting in the commit hook

[wthrowe]

We should check for lines ending with ] followed by lines starting with whitespace and ( in .md files, as these are probably links that got auto-formatted onto two lines, which doesn't work with doxygen. Checks requiring examining multiple lines are annoying to write, but we do it for NOLINT checks already, so something similar should be possible.

[nilsdeppe]

We could probably even just check for lines ending in ]. I think because that's markdown syntax, it's probably safe except in code blocks, but I don't know how common it is in those.