Update DocFX Version

[TimHess]

Currently uses or is compatible with docfx version 2.59.x (released Apr 10, 2022), but the current version is v2.75.2. Updating should bring access to bug fixes and enhancements (including a built-in dark mode)

[bart-vmware]

Please include a fix for #310 (comment) when working on this.

[TimHess]

This effort should also include making sure the 404 page is docfx-generated rather than hardcoded html

• Add a custom not-found page #357 (comment)