org-export-subtrees.el

(add-hook 'after-save-hook
          (lambda ()
            (org-map-entries
             (lambda ()
               (let ((name (nth 4 (org-heading-components))))
                 (when (and
                        ;; export only top-level entries
                        (= 1 (nth 0 (org-heading-components)))
                        ;; subtree export, body only
                        (org-html-export-as-html nil t nil t))
                   (goto-char (point-min))
                   (insert (format "<h1>%s</h1>\n" name))
                   (write-file (format "../web/templates/manual/%s.html"
                                       (downcase name)))
                   (kill-buffer)
                   )))))
          ;; just on this file
          nil t)