docs: add Foundations learning path — paced 3-day onboarding route

New src/docs/learning-path.mdx sequences the existing Foundations docs into
self-contained, bite-size lessons (Day 1 orient / Day 2 functional core /
Day 3 quality + capstone), each with goal, time, exercise, and self-check.
Insert it into the storySort order after Overview, add a pointer from the
Overview hub, and fix a stale cibg cross-link in atomic-design.mdx.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/docs/overview.mdx

@@ -8,6 +8,10 @@ The **why** behind this codebase, as a short curriculum. Each page is a condense
 cross-linked take on one idea; the long-form source lives in `docs/reference/`
 (see the repo's `docs/README.md`). Read them in roughly this order.
 
+> **New here?** Follow the [Learning Path](?path=/docs/foundations-learning-path--docs)
+> for a paced 3-day route with exercises and self-checks. This page is the map; the
+> Learning Path is the route through it.
+
 ## Architecture & domain
 
 - [Domain-driven design](?path=/docs/foundations-domain-driven-design--docs) — bounded