Revert scrollycode theming from docsite shell
The scrollycode theme support (scrollycode_theme_links helper and
--config scrollycode.theme=warm in dune-workspace) belongs in a
dedicated scrollycode shell, not in the generic docsite shell.
The generic --config infrastructure (config_values on Config.t and
the --config CLI arg) is retained for use by any shell.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>