Docs: Better handling of API changes across different API versions.

[Abscissa]

Related to #182, it would be nice to have a tool/mechanism by which no-longer-existing docs pages are replaced by a page that says something like "This symbol no longer exists in this version of the library. The last version it which had it was vX.Y.Z (linked to the appropriate archived page). It has been replaced with XYZ (link to appropriate page in current docs.)"

Further, it would also be nice for the documentation of older versions to be updated with things like "You are viewing an archived from an older version of this library. The latest version is vX.Y.Z (linked to same page in latest version of docs."

Summaries of the history of API changes would be nice, etc.

Also, could use ability to issue documentation updates/fixes for already-released versions.

Really, what's needed is a whole new documentation system, hopefully built on top ddox, or maybe forked from it.

[Marenz]

Possibly helpful and related: The release tool used by sociomantic: https://github.com/sociomantic-tsunami/neptune