doc: Add lib.meta to the library functions ToC

This seems like a pretty easy-to-fix oversight, and it documents the
getExe function, which I never knew about until one day I saw a PR
using it. Let's include it in the manual.

doc/default.nix

@@ -24,6 +24,7 @@ let
       { name = "cli"; description = "command-line serialization functions"; }
       { name = "gvariant"; description = "GVariant formatted string serialization functions"; }
       { name = "customisation"; description = "Functions to customise (derivation-related) functions, derivatons, or attribute sets"; }
+      { name = "meta"; description = "functions for derivation metadata"; }
     ];
   };