Support files can now either embed inline string content (Inline) or
reference a file path on disk to copy (Copy_from). This enables large
binary-like files such as x-ocaml.js (~16MB) to be managed via
`odoc support-files` without embedding them as OCaml string literals.

The scrollycode extension's x-ocaml script URLs are now configurable
via ODOC_X_OCAML_JS and ODOC_X_OCAML_WORKER environment variables,
and ODOC_X_OCAML_JS_PATH registers the file as a Copy_from support file.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- 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>

Add index.mld files with usage examples for each extension:

- mermaid: Flowcharts, sequence diagrams, state diagrams, etc.
- msc: Message Sequence Charts for protocol documentation
- dot: Graphviz/DOT diagrams for graphs and trees
- admonition: Note, warning, tip, important callout blocks
- rfc: IETF RFC citations with automatic linking

Each extension now has its own documentation page demonstrating
the syntax and showing live rendered examples.

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

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>