another common issue -- broken URLs

[yarikoptic]

just now got one on crossref. Those are even more annoying than typos since unlike typos you do not necessarily know how to "workaround" and need to google chase it all.

For that reason we already introduced linkchecking into bids-specification https://github.com/bids-standard/bids-specification/blob/9ad23e8ad14395b3471c8db1574c86739c7050c8/.circleci/config.yml#L39

It might need some more care to

• ensure that #fragments are still checked ok (I remember using my patched version, dunno if stock one still works good)
• ideally check external links in a smarter way -- only once in a while on cron, and on PRs only for the newly added ones (I don't think linkchecker is diff aware yet ;))