Fix extension rendering to not duplicate tag name
When an extension handles a custom tag, use an empty key in the
Description output so the tag name isn't shown twice. The extension's
output in the definition is self-describing.
Also remove redundant install stanza from deprecated/dune since the
parent doc/dune now handles these files via glob_files.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>