Try MkDocs `v1.5.0` link validation

[HonkingGoose]

Describe the proposed change(s).

The new MkDocs 1.5.0 version comes with better link validation. ¹ We'll first need to update to that version, and then I can try updating the config file, and test the site build.

Footnotes

1. MkDocs 1.5.0 changelog, expanded validation of links ↩