Add a JS plugin loading mechanism

[chalin]

Tracking issue for a JS plugin mechanism (target: 0.18): projects add or replace page scripts through assets/js/plugins/ plus a params.docsy.plugins entry, with no layout overrides.

• Plugin substrate: scripts.html decomposition + plugin loop (Add a JS plugin loading mechanism and decompose scripts.html #2788)
• Convert built-in integrations onto the loop (existing config keeps working) (Plugins: convert markmap, tabpane-persist, and click-to-copy; reshape the registry #2791)
• Document the plugin mechanism (config, asset convention, companions, exact field types) (Plugins: convert markmap, tabpane-persist, and click-to-copy; reshape the registry #2791)
• Opt-in interactive components built on the mechanism
• Decide head-side vs body-end emission for plugin stylesheets (body-end for now)
• 0.18 upgrade note: previously ignored override paths go live (_partials/algolia/head.html, and the new _partials/scripts/*.html names become overridable), and the internal named templates algolia/head and algolia/scripts no longer exist (replaced by the partials); sites calling or shadowing these will see the change