Merge remote-tracking branch 'jonludlam/extension-plugins' into staging-with-extensions

# Conflicts:
# dune-project
# src/document/comment.ml
# src/extension_api/odoc_extension_api.ml
# src/extension_registry/odoc_extension_registry.ml
# src/model_desc/comment_desc.ml
# src/odoc/bin/dune
# src/odoc/bin/main.ml
# test/integration/extension_plugins.t/run.t