Admonitions extension for odoc

Fix build against odoc learno-staging-rebase branch

- Add missing 'assets' field to extension_result records (rfc, admonition)
- Remove unused odoc_parser library dependency from dune (msc, mermaid, dot)

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

+1
+1
src/admonition_extension.ml
··· 78 78 (* Reference the external CSS file instead of inline *) 79 79 Css_url "extensions/admonition.css"; 80 80 ]; 81 + assets = []; 81 82 } 82 83 83 84 (* Register extension and its support files when plugin is loaded *)