CI: markdown link checker (#7145)

* change (CI): markdown link checker

* Fix some invalid doc links (re-run of cargo-unleash gen-readme w/ fixes).

* Fix some invalid doc links

* Fix some invalid doc links

* Fix some links

* Fix some links

* Apply @bkchr

 suggestions from code review

Co-authored-by: Bastian Köcher <[email protected]>

* Fix more links

* Fix more links

* typo

* Fix more links

* Fix more links

* Ignore valid link .. check wrongly sees it as invalid

* Fix style issue

* Fix style issue

* change (CI): update style guide link

* change (lib): suggestions

Co-authored-by: Dan Forbes <[email protected]>
Co-authored-by: Steve Degosserie <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>