Fix odoc plugin dynamic linking with -linkall flag

When Dynlink tried to load extension plugins (like rfc_extension.cmxs),
they failed with "undefined symbol: camlOdoc_extension_api" because the
odoc binary didn't export symbols needed by the plugins.

Adding -linkall to the odoc executable ensures all modules (including
odoc_extension_api) are linked into the binary and their symbols are
available for dynamic linking.

This enables extension plugins loaded via dune-site to work correctly.
Tag handlers (rfc, admonition) and code block handlers (dot, mermaid, msc)
now load and process tags during HTML/JSON generation.

Co-Authored-By: Claude Opus 4.5 <[e-mail]>

src/odoc/bin/dune

···
  (libraries cmdliner odoc_model odoc_odoc odoc_extension_api
             dune-site dune-site.plugins)
  (modules main sites)
+ ; -linkall ensures all modules are included for plugin dynamic linking
+ (link_flags (-linkall))
  (instrumentation
   (backend landmarks --auto))
  (instrumentation