tappet.in (make-html-*): Add docstring.

* tappet.in (make-html-*): Add docstring.
  (%cadrobot): Rename to `%cadrobot-url'.
  (%tappet): Rename to `%tappet-url'.
tappet.in (write-html-page!): New procedure

* tappet.in (write-html-page!): New procedure.
  (write-index!, export-log!): Use it.