doc: set default font, optionally

doc/1.intro.md

@@ -105,11 +105,6 @@ references:
         given: H.
     issued: 2003-3-5, 2003-4-5
 
-# Font
-# mainfont: Libertinus Serif
-# mathfont: Libertinus Math
-# monofont: Fira Mono
-
 # PDF output options
 classoptions:
   - a4paper
@@ -124,6 +119,14 @@ css: res/style.css
 # pandoc-crossref options
 linkReferences: true
 tableEqns: true
+
+header-includes: |
+    ```{=latex}
+    % Set font only if available
+    \IfFontExistsTF{Libertinus Serif}{\setmainfont{Libertinus Serif}}{}
+    \IfFontExistsTF{Libertinus Math}{\setmonofont{Libertinus Math}}{}
+    \IfFontExistsTF{blabla}{\setsansfont{blablabla}}{}
+    ```
 ...
 
 [^1]: Istituto per la Scienza e Tecnologia dei Plasmi,