PHP 8 and Diviner Technical Documentation

Summary:
This also fixes the field Personal Settings > Display Preferences > Monospaced Font, when empty.

Close T15621

Test Plan: Generate diviner stuff, navigate to the "Technical Documentation" section (In php 8.1+)

Reviewers: O1 Blessed Committers, mturdus, valerio.bozzolan

Reviewed By: O1 Blessed Committers, mturdus, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15621

Differential Revision: https://we.phorge.it/D25412