Issue #3607921: Docs: external-interaction internals page, nav grouping, and notifications split

Documentation improvements for the external-interaction subsystem and the docs site structure. Three related parts:

1. New illustrated page: External interaction internals (external-interaction-internals.md), the code-level deep dive behind the External interaction overview (which only summarizes it). It covers the request lifecycle, where the first link comes from (the generic orchestra_interaction.urls primitive first, then the shipped conveniences as examples, including the custom "Book button" pattern), the capability-token anatomy, minting/validating, the dispatcher, link scope, a threat-by-threat security table, the two-doorway model, and the Webform integration. Mermaid diagrams plus inline static SVG (no extra JavaScript). A "Going deeper" link is added from the overview page.

2. Docs nav grouped into sections. The flat list of ~28 first-level pages is grouped into Concepts, Modeling workflows, External interaction, Running workflows, Integrations and extension, Observability, and Project (using the theme's existing navigation.sections).

3. Notifications split into two pages. notifications.md now covers the modeling side (where/who/what, the notification points, audiences, the Notify node) and sits under Modeling workflows; a new notification-delivery.md covers the delivery side (the channel-neutral event, orchestra_mail, orchestra_easy_email, customizing/replacing, SMS/chat/push via the Recipient API) and sits under Integrations and extension.

All pages verified with mkdocs build --strict; referer added to the cspell word list.