Check status badges

[jeroen]

Added a badge that tries to summarize the "check status" of the package, e.g:

https://ropensci.r-universe.dev/magick/badges/checks

It is somewhat arbitrary which runs are taken in consideration when summarising results into a single "conclusion".

The current implementation includes status for building vignettes and check results for r-release and r-devel on win/mac/linux and then shows the "worst result" of those checks. So if there is 1 "error", 1 "warning and 5 ok, the badge shows "error".

It ignores r-oldrel and platforms where we only build binaries but do not check (webassembly), to avoid false positives.

We could also split out the badges: one with the check result on windows, one for macos, one for linux. Or by R release. But I am not sure how useful that would be.

[jeroen]

@maelle if this looks good we can document the api.

[maelle]

To me the current API is good as is, because it matches my expectations of GHA status badges. I'll make a PR to the docs.