link checking should include [ref page](ref:slug) syntax in addition to [guide page](doc:slug) link syntax

[fscelliott]

this sync tool should enforce fixing links from the guides to the API reference that have the internal link ref page syntax, but seems to miss them? I tested by inserting [test ref](ref:test-me-broken)](ref:test-me-broken) and the tool synced successfully. It fails if I insert [test doc](doc:broken-link)](doc:broken-link).

tested in this commit https://github.com/sensible-hq/sensible-docs/actions/runs/5719311414/job/15496934038

[fscelliott]

I haven't checked, but if the sync tool doesn't enforce link validation for (changelog page)[changelog:slug] type syntax it should as well.