docs: review pass on remaining docs (principles, reference, examples, guides)

[tri2510]

Summary

Code-verification pass over the remaining docs/ areas not covered by #538 (getting-started) or #539 (architecture): principles/, reference/, examples/, guides/ (incl. the plugin guide), and docs/README.md.

Headline finding: unlike the architecture docs (~95% accurate), several "remaining" docs were significantly outdated or fictional. This PR corrects the factual errors and rewrites the worst offenders to match the current code. No code changed — documentation only.

Rewrites (to the current code)

• principles/project-structure.md — full rewrite to the real layout (backend/src/routes/v2/{user-management,vehicle-data,content,system}, backend/src/index.js, backend/static/frontend-dist/, backend/Dockerfile, instance-setup/). The old doc described a v1 layout (routes/api/*-management, a top-level frontend-dist, a root Dockerfile).
• guides/plugin/04-api-reference.md — completed. The Plugin API has 28 methods across 8 categories (not 14). Added Asset Operations (8) and Kit/Runtime Operations (4) sections, fixed both PluginAPI interface blocks (the "complete" one even missed uploadFile), and documented the editable prop.
• reference/component-design/dynamic-components.md + docs/examples/* (5 files) — replaced the fictional "Dynamic Component Architecture" (ComponentRegistry / renderComponentByTypeName) and fictional examples (Banner, ModelsGrid, useMyModels, …) with the real implementation: PageHome's config + getComponent switch, route-level React.lazy + retry(), and real components/hooks/pages (HomeHeroSection, DaModelItem, useCurrentModel, PageModelList).

Factual fixes

• plugin/02-architecture.md — registration timeout 5s/50 → 15s/150 (matches PluginPageRender.tsx).
• plugin/README.md, plugin/03-creating-plugin.md — broken PLUGIN_API_POSSIBILITIES.md link → ../plugin-api-possibilities.md.
• reference/feature-breakdown.md — corrected stale builtin routes (/user/asset→/my-assets, /admin/manage-user→/admin/manage-users, /models/:mid/prototypes/:pid→/model/:model_id/library/prototype/:prototype_id).

Noted, not changed (would touch code or need a decision)

• reference/csp.md — the shipped CSP (app.js) is wildcard-permissive, not the tight policy documented here; the doc now says so. Tightening the shipped CSP is a code/product change, kept out of this docs PR.
• guides/plugin-api-possibilities.md — several capabilities listed as "future" are already implemented; noted inline.

Verification

• Broken-link target confirmed absent; ComponentRegistry / useMyModels / Banner confirmed absent via grep in frontend/src.
• Plugin API signatures quoted verbatim from frontend/src/types/plugin.types.ts; example code quoted from the real files (PageHome.tsx, useCurrentModel.ts, DaModelItem.tsx, HomeHeroSection.tsx, configs/routes.tsx).
• global.css is at backend/static/global.css; there is no root Dockerfile/docker-compose.yml.

Notes

• Commits signed off (Tri Hua <tri2510@gmail.com>).
• Independent branch off main; does not depend on getting-started onboarding: docs + local-dev config + unblock backend tests (new-user review) #538 or docs(architecture): code-verify fixes (counts, rate-limit, token lifetime, startup, toJSON) #539.