docs: fix copyright years for Sphinx pages
docs: fix copyright years for Sphinx pages

In further evidence that computers were a mistake, because `nix` sets
`SOURCE_DATE_EPOCH=1` when it builds things and because Sphinx has a
workaround for bugs in _different_ packages that might be doing
something silly with dates, we need to explicitly set
`SOURCE_DATE_EPOCH` so we don't end up 40+ years in the past.

https://github.com/sphinx-doc/sphinx/issues/3451

cpcloud/ibis,ibis-project/ibis,cpcloud/ibis,ibis-project/ibis,ibis-project/ibis,ibis-project/ibis,cpcloud/ibis,cpcloud/ibis