[ci] move link checking to a cron job

[jameslamb]

Based on the conversation in #3785, this PR proposes moving linkchecker checks to a cron job.

The link checking part of check-docs is one of the most common sources of instability in LightGBM. In my experience, failures in this job are almost always temporary disruptions like a server complaining about rate limits, and those failures are almost always solved by just re-running it.

Moving this task to a cron job, instead of running it on every commit to master or a PR, lessens the burden on maintainers of manually re-running this task and improves the contributor experience by removing a check that might fail in ways unrelated to their contributions. It also reduces the load that LightGBM puts on those servers that serve the things we link to.

Testing

Ok after a few small changes, this appears to have worked.

• link-checker job: https://github.com/microsoft/LightGBM/pull/3890/checks?check_run_id=1811573220
• check-docs: https://github.com/microsoft/LightGBM/actions/runs/529538824

Notes for Reviewers

This new workflow can be triggered on the "Actions" tab in the GitHub UI, but only after this PR has been merged to master: https://github.community/t/workflow-dispatch-event-not-working/128856/2.

[StrikerRUS]

Thanks for doing this!
Just two minor comments.

[github-actions]

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