draft: True not actually implemented

[eichin]

doc/reference/fields/draft.md describes an explicit draft flag, as an addition to the "date in the future" aspect. I find this a lot more clear than "magic future date", especially in terms of actually having an "ok I'm really done" commit point. Unfortunately, it's not actually implemented; create_source_page_as_file has a # Skip draft pages section, which only checks the date and the --draft flag, but nothing else. (Also, I accidentally published a page because I only had draft: True in the top-of-page YAML block, but adding date: 2038-01-01T00:00:00 and rebuilding unpublished it.)

(Consider this the tiniest of votes for actually adding the feature, but I completely understand just dropping it from the docs since there is a working mechanism.)