Site review fixes: SPA navigation, broken demos, merlin config

Dot extension: Replace per-diagram Js_inline scripts with a single
Js_url support file (dot-init.js) using MutationObserver pattern.
Store DOT source in <script type="text/dot"> elements to avoid
escaping issues. Fixes SPA navigation rendering.

Merlin config: Add stdlib path to build_path so dynamically loaded
packages (e.g. cmdliner via #require) resolve in merlin's type
checker. Previously only stdlib was set, causing red squiggly
errors on library references despite code compiling fine.

Widget_leaflet: Re-export Leaflet_map module from Widget_leaflet
so it's part of the public API. Add leaflet widget demo page.

Interactive extension demos: Move broken demos to .notyet:
- demo2_v2/v3: fake v1/v2 distinction (same cmdliner version)
- demo3_oxcaml: comprehensions extension disabled in compiler
- demo4_crossorigin: requires jon.ludl.am infrastructure
- demo5_multiverse: requires localhost:9090
- demo6/demo7: porting workshops need work

Fix demo_map and demo_widgets: add @x-ocaml.universe and
@x-ocaml.worker tags pointing to /_opam.

Simplify build-site.sh: remove redundant v2/v3 universe builds.

Sidebar: Add odoc-md to 'odoc Core' package group.

Scrollycode: Remove theme references from docs, align titles.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>