GitHub action to check for broken URLs

[Aariq]

Adds a GitHub action that runs lychee to check for bad URLs every monday and on pull requests.

When it is run as a cron job (or with a button-click via "workflow dispatch") it will open an issue regarding broken URLs. When triggered by a PR it will simply fail without opening an issue (assuming that the PR introduced the bad URLs and they should be fixed before merging it).

Would appreciate feedback on how often this should run. Perhaps only monthly?

To do:

• [x] deal with private project board URLs (either ignore github.com/orgs/cct-datascience/projects or create a custom token with permissions to see these)
• [x] deal with private google docs and hackmds
• [x] only check source documents, not _book/

[Aariq]

Checks are failing because there are many broken URLs! I suggest merging this first despite the failing check, then fixing broken URLs in a separate PR.