Use `buffer-face-mode` in adoc-mode

This replaces the use of `variable-pitch-mode`, which prevented fixed
pitch for, e.g., code blocks.

user/emacs.nix

@@ -158,7 +158,7 @@ in
           (adoc-mode . (lambda ()
                          (visual-line-mode)
                          (visual-fill-column-mode)
-                         (variable-pitch-mode)))
+                         (buffer-face-mode)))
         ''];
         config = ''
           (set-face-background 'markup-verbatim-face nil)