[DEPRECATE] Removal of backward-compatibility of some #docinfo items

[Rob Beezer]

Before we hit on the idea of a publisher's file for configuration of non-content
aspects of a project, we allowed options to be given in #docinfo. As we have
migrated some of those options, we have provided deprecation messages, but also,
at little cost, we kept old code around so that publishers could make changes at
their leisure.

Tht cost of some of these luxuries has now gone way up, and some backward
compatibility features need to be removed. All are at least two years old, and
they mostly affect HTML.

Here is a (rough) list of items that once were given in #docinfo, whose
specifications were honored by backward-compatible code, and which now will just
be ignored. Dates are the time of the original deprecation. Note that you will
get a fresh deprecation message saying specifications are now being ignored
(rahter than saying we will "honor your intent").

Why? Oscar L, Andrew S, and I have been working on speeding up the necessity of
examining the publisher file as part of generating assets associated with a CLI
build of a project. This is a first step in that process.

Rob

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2019-04-14
docinfo/analytics: StatCounter, Google "classic", Google "universal"

2019-04-19
docinfo/search: Google "cx" style search

2020-11-22
docinfo/html/baseurl

2023-01-07
docinfo/feedback: "custom" feedback button

2023-01-10
docinfo/covers: PDF versions, @front, @back