Javadoc: monospace font fallback

[slauth]

I'm on Ubuntu and have none of the listed fonts installed. All code blocks are therefore rendered with a serif font which looks rather ugly. Adding the generic monospace family as fallback fixes this.

[TimvdLippe]

Nice fix!

[codecov-commenter]

Codecov Report

Merging #2434 (e0fc246) into main (5653127) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #2434   +/-   ##
=========================================
  Coverage     86.40%   86.40%           
  Complexity     2761     2761           
=========================================
  Files           318      318           
  Lines          8299     8299           
  Branches       1015     1015           
=========================================
  Hits           7171     7171           
  Misses          854      854           
  Partials        274      274           

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