fix(docs): set mode in file-local vars

Due to emacs-straight/org-mode@e22b4eb, a replacement major mode (for
org-mode) can no longer be specified from .dir-locals.el, as it spirals
into infinite recursion. Therefore, a mode: line is needed in all Doom
docs.

Ref: emacs-straight/org-mode@e22b4eb7aa

core/autoload/docs.el

@@ -40,7 +40,10 @@
   (let ((beg (point-min))
         end)
     (org-with-wide-buffer
-     (goto-char (point-min))
+     (goto-char beg)
+     (when (looking-at-p "^# -\\*- ")
+       (goto-char (line-beginning-position 2))
+       (setq beg (point)))
      (when (looking-at-p org-drawer-regexp)
        (re-search-forward org-drawer-regexp nil t 2)
        (goto-char (setq beg (1+ (line-end-position)))))