Link checker Github Action runs too many times and opens too many issues

[valeriupredoi]

• it appears it runs for every push to branch if docs are changed in that branch/PR https://github.com/ESMValGroup/ESMValTool/actions
• it opens a lot of issues

@flicj191 if you don't mind me, I'll throttle down the output in a PR, and will ask you for review and opinions on how best to have it run 🍻

[valeriupredoi]

OK for now I disabled the link checker in PR workflow, notwithstanding because tj-actions/changed-files had a serious security breach about nine months ago https://www.stepsecurity.io/blog/harden-runner-detection-tj-actions-changed-files-action-is-compromised and tj-actions/changed-files#2463

It'd be good if we could find a replacement for that action - it appears to be safe now, and GitHub has indeed reinstated it, but you never know when the bad actors decide to pay a second visit.

Also, it'd be very useful to have the action run only if certain strings changed in the docs ie stuff that contains "http" or "https", otherwise that action runs everytime there is any push to any of the rst docs, potentially hundreds of times per PR - my problem is not that it is wasteful - Github has enough resources for a small action to keep running, my problem is that I need to go back 4 or 5 pages to find the nightly test runs