doc: Update links which returned 404

doc: Update links which returned 404

Two links in the isn module documentation were pointing to tools
which had been moved, resulting in 404 error responses.  Update
to the new URLs for the tools.  The link to the Sequoia 2000 page
in the history section was no longer working, and since the page
is no longer available online update our link to point at the
paper instead which is on a stable URL.

These links exist in all versions of the documentation so backpatch
to all supported branches.

Author: Daniel Gustafsson <daniel@yesql.se>
Reported-by: charukiewicz@protonmail.com
Discussion: https://postgr.es/m/173679670185.705.8565555804465055355@wrigleys.postgresql.org
Backpatch-through: 13