Add automated link checker

[lukruh]

Description

This PR runs the lychee link checker as scheduled GH-Action once a week.
If broken links are found an Issue with the report is created automatically (see #98).
It's following the approach from @flicj191 in ESMValGroup/ESMValTool#4251. With some adjustments for the jekyll based website.

Certain domains (like doi.org) block automated requests. To avoid reporting such urls as broken every week, they can be added to the .lycheeignore file.

To be done before merge:

• run separate actions for new PR (only changed source) and weekly (expansive production build)
• improve the report issue (less verbose, provide some guidance to solve it)
• remove the test trigger on push
• add date to title
• automatically assign a "website" team rather than someone specific? (what do you think @axel-lauer?)
• test scheduled (should trigger Sunday night), only works in main branch

Closes #13

Example issue created by the weekly check: #104
Example PR that adds a broken link and fails the PR test: #108
The actual fixes of broken links is done in #109

Checklist

It is the responsibility of the author to make sure the pull request is ready to review.

• New functionality is working (e.g. links, images showing correctly, etc.)
• Content is appropriate
• Spelling and grammar are correct
• All checks below this pull request were successful