Re-enable documentation link checking

[Archmonger]

Description

Documentation link checking via linkcheckmd was disabled due to ReadTheDocs (RTD) blacklisting GitHub Workflows IP addresses. At the time, there wasn't any viable workaround besides either disabling link checking or removing all links to RTD.

A potential solution will involve switching over to linkspector and adding RTD links to the ignored URLs, but linkspector is currently critically broken.