CI check for changelog markdown syntax

[Nathan van Doorn]

As discussed in #2286, the changelog file often ends up with invalid markdown. We should add a CI check to ensure that it remains valid.

[jamesmckinna]

@andreasabel echoing your comment on #2325

There's no reason for the CHANGELOG not to be under whitespace fixes.

Technically, trailing whitespace in .md files stands for <br/>, but I think this is a misfeature that shouldn't be used. (Use <br/> directly if you must.) Another reason I can think of that folks want to use tabs, but again, one can instead use spaces.

Would you have any good suggestions as to the best way to improve CI for the CHANGELOG to do (full) markdown validation?

[MatthewDaggitt]

@jamesmckinna something like this should work: https://github.com/marketplace/actions/markdown-lint