Fix building the online version of the manual.
* doc/Makefile.am (GENDOCS): Use the version from the tar repository.
(manual): Set the DISTRIN rendition.
(manual-rebuild): New goal.
* doc/README.manual: New file. Instructions for maintainers on how to
update web documentation.
* doc/gendocs.sh: A version from gnulib fixed as per
https://lists.gnu.org/archive/html/bug-gnulib/2021-03/msg00002.html.
* doc/gendocs_template: Updated version from gnulib.
* doc/intern.texi: Fix the use of UNREVISED.
* doc/tar.texi: Fix the use of GNUTAR.