Support a bidder documentation archive

[bretg]

We discussed in the docs committee that there ought to be a way of archiving PBJS and PBS bid adapters that are no longer active.

We need the docs to stay available in case they're not actually dead, but if they haven't been ported to a recent release, publishers would like for the bid adapter pages to be less cluttered.

This is separate from the "enable_download" flag, as it's conceivable that an adapter may want to be active but paused for new adoption.