Phase 2: restructure the Concepts (Reference) docs

[developerjamiu]

Restructures the Concepts section (public label "Reference") into a predictable, findable shape: eight journey-ordered groups, plus a root-level Reference section for true lookups. Legacy content moves to Upgrading/Archive, and duplicated facts get a single home. It ships as seven reader-coherent PRs, and the section rename to "Concepts" happens last.

Acceptance bar (every PR)

Each PR is done only when every page it touches, including pages that mostly stay put, passes:

• Template conformance: capability pages have a substantive Overview plus action-shaped Operations; reference pages are terse and complete with stable anchors; procedural pages have action-shaped steps, prerequisites, and a verifiable end state.
• Flow: reads top-to-bottom as a first-timer, with no forward references.
• Self-contained: every term is explained on first mention; no reliance on an adjacent page having been read.
• Gap coverage: folds in the relevant open issues and correctness fixes for the area it touches; anything not folded is filed and named in the PR.
• No information loss: every unit is preserved or explicitly archived; the build and links pass.

PRs

• PR 1: Server fundamentals, the reference spine, and the archive scaffold (Reduce duplication: make 00-start-command.md concept-like, link to CLI reference start page #664, Update references to 'serverpod generate' to reflect the changes with hot reload #538, Address all mentions of dart run bin/main.dart #565)
• PR 2: Endpoints and APIs (Add Client-side error handling + practical streaming examples to framework docs #648, Documentation: Clarify approach for header setting/getting documentation #367, Document client URL configuration per environment and handling expired auth #654)
• PR 3: Data and the database (Expand many-to-many docs: examples, why/when, and troubleshooting #652, Add ID-strategy guidance: int vs UUID, autoincrement, client-set IDs #657, Improve migrations docs: custom SQL, Serverpod Cloud apply guidance, troubleshooting #659, epic: Improve the database docs #504)
• PR 4: Authentication, plus a migrate-from-legacy-auth guide (epic: Improve the auth docs #503, Improve the Facebook sign-in documentation #507, Improve the Microsoft sign-in documentation #508)
• PR 5: The web server and scheduling (Enhance body streaming documentation in Request Data section #365, Add sample code for Jaspr alternative in server-side HTML documentation #366, Fill future-call gaps: timeouts, DB inspection, scheduling from an endpoint #655)
• PR 6: Operations and testing (Move the Contribution Testing Guidelines to framework docs #629)
• PR 7: Rename to "Concepts", promote the Reference section, redirects and sidebar (Move away from using file-name ordering in framework docs #587)

Notes

• The CLI reference is now auto-generated (#5390), so these PRs link to it rather than rebuild it.
• Items that need framework-team confirmation are deferred to a later pass so they do not block this work: database seeding (Document database seeding (insert initial data at startup) #651), server role behavior, and the passkey UI. Structural work around them still ships.