docs: Group Concepts into eight sections and rename the section to Concepts

[developerjamiu]

Final PR of the Concepts Phase 2 rollout: the structural pass. PRs 1–6 reshaped page content in place; this one moves everything into the agreed information architecture ("rename last").

What changed

• Section relabeled Reference → Concepts (label edit in _category_.json; the 06-concepts folder does not move).
• The flat page list is grouped into eight sections, in reader-journey order:
  1. Server fundamentals (running your server, configuration, modules)
  2. Endpoints & APIs (working with endpoints, sessions, inheritance, HTTP config, middleware, error handling, streaming, server events, file uploads)
  3. Data & the database (models + database subgroups)
  4. Authentication
  5. Web server
  6. Scheduling
  7. Operations (caching, logging, health checks, security & TLS, experimental features)
  8. Testing
• Sessions now lives under Endpoints & APIs, right after "Working with endpoints" — it is defined entirely in terms of endpoint methods, so it must follow them (this restores the pre-rollout order).
• cli and lookups stay under Concepts as subgroups (the CLI tree is frozen — the sync bot writes to the hardcoded docs/06-concepts/cli). Their sidebar-icon-tools icons were removed so all Concepts subgroups are visually consistent.
• 139 files moved via git mv (history preserved). 142 internal links were rewritten to the new structure; the build passes with onBrokenLinks: 'throw'.
• Content is link-only — no prose changed.

Deliberate deferrals

• Legacy content stays in place (legacy auth, legacy scheduling) — not archived in this PR.
• Redirects are deferred to the 4.0 GA cutover. The new URLs exist only in /next; the site serves 3.4.0 at the root, where the plugin rejects the new paths as invalid to targets. No stable URL is stranded (3.4.0 is frozen; all /next internal links are fixed). The 50 redirects are prepared and will be added when 4.0 becomes the default version.

Follow-ups (separate PRs)

• Modernize serverpod generate / dart run bin/main.dart references around the hot-reload dev loop (Update references to 'serverpod generate' to reflect the changes with hot reload #538, Address all mentions of dart run bin/main.dart #565).
• Consolidate the Authentication get-started/setup entry pages.