doc: better font stack for monospace in docs

There appear to be rendering issues with inconsistent glyph width when
using the Monaco font on non-macOS machines. This change updates the
native CSS font stack to use the same font stack as Boostrap does, minus
the Monaco font.

PR-URL: nodejs#21036
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yazhong Liu <yorkiefixer@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api_assets/style.css

@@ -35,7 +35,7 @@ h1, h2, h3, h4, h5, h6 {
 }
 
 pre, tt, code, .pre, span.type, a.type {
-  font-family: Monaco, Consolas, "Lucida Console", monospace;
+  font-family: SFMono-Regular, Menlo, Consolas, "Liberation Mono", "Courier New", monospace;
   font-size: .9em;
 }