Add script for checking broken links in website

**Is your feature request related to a problem? Please describe.**
guild-website has a nice script for verifying broken links

**Describe the solution you'd like**

- add the same script as we use in guild-website https://github.com/the-guild-org/the-guild-website/blob/master/check-links.ts
- adapt to graphql-tools if needed
- setup GitHub workflow to trigger `check-links` script

**Describe alternatives you've considered**

**Additional context**

Start by reading the referenced guild-website check-links.ts script and inspect graphql-tools' website and workflow configuration. Adapt the script where graphql-tools differs, then add a GitHub workflow that triggers check-links. Done means the workflow runs successfully and reports broken website links.

Written by the indexing model from the issue text.