Also traverse preamble to check for math presence

Signed-off-by: Paul-Elliot <[email protected]>

Author: panglesd

CHANGES.md

@@ -27,6 +27,7 @@
   compilation unit. This fixes cases where the dune rules would
   fail. (@panglesd, #1069)
 - Fix issue #1066 with extended opens (@jonludlam, #1082)
+- Fix missing katex headers (@panglesd, #1096)
 
 
 # 2.4.0

src/document/doctree.ml

@@ -369,7 +369,7 @@ end = struct
 
   and subpage : Subpage.t -> bool = fun x -> page x.content
 
-  and page : Page.t -> bool = fun x -> items x.items
+  and page : Page.t -> bool = fun x -> items x.preamble || items x.items
 
   and alternative : Alternative.t -> bool = function
     | Expansion x -> documentedsrc x.expansion