Requested by @Bugreporter2 at T202424#9312250

Feature summary (what you would like to be able to do and where):
Pages with content model = "Plain text" should use a monospaced font instead of a variable-width font.

As https://en.wikipedia.org/wiki/Monospaced_font says:

"Even though computers can now display a wide variety of fonts, the majority of IDEs and software text editors employ a monospaced font as the default typeface."

This is the expected display for a plain text page.