Add a workflow to regularly run `make linkcheck`

This is a follow-up of #727 and related to #681. Since make linkcheck is somewhat unreliable and can't be required for regular PR, the idea is to use a workflow to run it regularly and create (or update) an issue when a link is unreachable. Note that make linkcheck is already run for PRs, but the result is only visible by looking at the logs.

@AlexWaygood shared a similar workflow in https://github.com/python/devguide/pull/727#issuecomment-1100939982:

Over at typeshed, we have a test that runs every night. If it fails, a bot auto-creates an issue for us. It works pretty well!

Here's the .yml file: https://github.com/python/typeshed/blob/master/.github/workflows/daily.yml

And here's an example of an issue that the bot auto-created when the test failed: https://github.com/python/typeshed/issues/7572

This workflow creates a new issue every time, and it might be ok if it's run weekly/monthly however -- if we want to run it more frequently -- updating an existing issue might be a better approach.

@hugovk also raised a concern in https://github.com/python/devguide/pull/727#issuecomment-1100912440

One concern: if the cron is often failing (which it might, and often), we don't want it to show the repo as failed at the top:

If this sounds useful we can agree on the details, fix the currently broken links, and adapt the workflow linked above.

Inspect the existing PR workflow that runs make linkcheck and compare it with the referenced typeshed .github/workflows/daily.yml. Decide whether the scheduled workflow should create new issues or update an existing one, and ensure link failures are reported without making the repository's regular PR checks fail.