Document intentionally skipped citeproc tests

[PgBiel]

https://github.com/typst/hayagriva/issues/327 lists failing citeproc tests we may want to actually fix.

We should also list citeproc tests which we intentionally don't want to fix, such as date_DateBC (as noted here: https://github.com/typst/hayagriva/issues/327#issuecomment-2986240675), including a reason (in this case, because we treat negative years with an offset of 1 per conventions).