Autogenerate licence text in doc subdir from LICENCE.

Now we have licence.pl, it seems to me to make very good sense to have
it generate the Halibut form(s) of the licence and copyright year as
well as the source-code forms.

As a result, I believe _no_ copies of the licence text or copyright
date exist any more except for the master one in LICENCE - so I can
completely remove the checklist section about all the places to update
it, because there's only one. Hooray!