doc: remove files left over from pre-Antora doc builds

The doc directory still carried the hand-written reference pages and the
Asciidoctor assets from the toolchains that preceded Antora and MrDocs.
Antora reads only doc/antora.yml and doc/modules, so none of it was
reachable from the build; a full build before and after these deletions
produces byte-identical output except as noted below.

Removed:

- 29 top-level *.adoc reference pages, superseded by the MrDocs-generated
  reference and the pages under doc/modules/ROOT/pages.

- docinfo.html and the zajo-{light,dark}.css it pulls in, copied from
  Boost.LEAF (the meta description still advertised "Lightweight Error
  Augmentation Framework"), plus rouge-github.css. Antora supplies its own
  styling and does not read docinfo files.

- gentags.sh and doc/tagfiles. The antora-cpp-tagfiles extension loads
  tagfiles named in ext.cpp-tagfiles.files, which antora.yml does not set,
  plus the cppreference tagfile it now bundles itself; gentags.sh wrote to
  a file name that was not even present in tagfiles.

- openmethod-theme.yml, an asciidoctor-pdf theme with no PDF build left,
  and the unreferenced skin.png and open.png.

Also removed two orphan pages under doc/modules/ROOT/pages. These were in
the nav of no page but Antora publishes pages regardless, so both did
render: error_processing.adoc was empty, and virtual_parameter.adoc was a
stub titled "Macros" whose body showed unconverted Doxygen @ref markup as
literal text. Dropping them removes two URLs from the sitemap and two
entries from the search index.

Kept library-detail.adoc, which is not in the Antora nav but is the Boost
website's library page, and doc/modules/ROOT/images.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: jll63