Publish `check_docs.ts` script as a package

**Is your feature request related to a problem? Please describe.**

Recently the Fresh CI broke because the checksum of https://github.com/denoland/std/blob/main/_tools/check_docs.ts changed, see https://github.com/denoland/fresh/actions/runs/17701967986/job/50308802072?pr=3425

**Describe the solution you'd like**

Since this script is popular among our repositories we should publish it as a proper package instead.

**Describe alternatives you've considered**
-