add link check action

[guan404ming]

may use like

• https://github.com/lycheeverse/lychee-action
• https://github.com/marketplace/actions/check-links-with-linkcheck

[jorenham]

will that also work with mkdocs and https://facelessuser.github.io/pymdown-extensions/extensions/magiclink/?

[guan404ming]

I could help look into it!