Fix build and doc warnings, remove dune subtree

Build fixes:
- Change odoc extension library refs to odoc.extension_api
- Add missing assets field to extension result records

Documentation fixes:
- Fix cross-package references using documentation dependencies
- Fix reference syntax: {!Stdlib.String.escaped}, {!Webfinger.Jrd}, etc.
- Fix heading levels and anchor syntax (#→.)
- Add @canonical annotation for Loc0.t
- Escape braces in doc comments
- Add asset support via (files ...) in documentation stanza

Infrastructure:
- Remove dune subtree (now pinned to jonludlam/dune#odoc-assets-support)
- Add dune-workspace with fatal odoc warnings
- Remove conflicting odoc-config.sexp install stanzas

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>