org: better display for CSV tables

* doc/org/.dir-locals.el.in,
doc/org/init.el.in (org-html-table-header-tags): Define.
* doc/org/spot.css: Style the tables.

doc/org/.dir-locals.el.in

@@ -40,6 +40,8 @@
 	      (shell-file-name . "@SHELL@")
               (ess-ask-for-ess-directory . nil)
               (org-export-html-postamble . nil)
+              (org-html-table-header-tags .
+                 '("<th scope=\"%s\"%s><div><span>" . "</span></div></th>"))
               (org-babel-default-header-args:plantuml
                . ((:results . "file")
                   (:exports . "results")